Preheat your oven to 350°F/175°C. Butter or grease three 8-inch baking pans and line it with parchment paper. If you only have two pans, bake the first two cake layers and let them cool slightly before reusing one of the pans for the third layer.
In a stand mixer fitted with the paddle attachment, combine the flour, cocoa powder, sugar, baking powder, baking soda, and salt.
In a large jug or bowl, whisk together the buttermilk, oil, eggs, and vanilla extract.
2 eggs, 1 cup buttermilk, ½ cup vegetable oil, ¾ teaspoon vanilla extract
While the mixer is on your lowest speed, slowly pour in the wet ingredients into the dry.
While the mixer is still running, slowly add the hot water and mix. Your batter will be very thin, so don't worry! Make sure the mixer is on your lowest speed so it doesn't splash everywhere!
1 cups very hot water
Pour the batter into the prepared pans, it's about 470 grams per cake pan. Bake for 22-25 minutes, until a toothpick comes out clean when inserted
Allow the cakes to cool for 10 minutes before removing from the pans. Let them cool completely on a cooling rack.
Make Filling
In a medium saucepan, combine the diced strawberries with the sugar, lemon juice and lemon zest. Heat over medium-low heat and bring to a simmer.
2 cups strawberries, 4 tablespoons white sugar, 1 ½ tablespoons lemon juice, lemon zest
Simmer for 25-30 minutes, stirring frequently. You may need to lower the heat to low heat if it is bubbling too much, you don't want it to burn.
You'll know the strawberry sauce is ready when you can coat the back of a spoon with the sauce and run a line through it with your fingers. Take it off the heat and pour into a large jar or bowl to cool completely. Place in the fridge.
Make Frosting
Fill a medium saucepan with an inch of water and bring to boil. Combine the sugar and egg whites in a large mixing bowl (if you have a stand mixer, you'll want to use that bowl), set the bowl above the simmering water.
4 large egg whites, 1 cup white sugar
Heat, whisking constantly, until the sugar dissolves. The mixture will go from a yellowy, granular substance to white and opaque, this takes about 5 minutes. I recommend taking a pastry brush to brush the sides of the bowl so all of the sugar is getting dissolved. Using a thermometer, it should read 140°F/60 C. If you don't have a thermometer, when you pull the whisk up feel the mix with your fingers – if it is no longer grainy then it is ready for the next step.
Put the bowl into your stand mixer with a whisk attachment and beat on medium-high until you get a glossy meringue with stiff peaks – about 10-15 minutes. Note it can take up to 20 minutes if the weather is hot and humid. You'll want to be beating the mixture until the bowl is completely cool. If the bowl or meringue is too warm it will melt the butter.
Switch to paddle attachment. Add butter one piece at a time, allowing it to incorporate fully, while still at medium-high speed. This should take 8-12 minutes. When you've added all the butter and the mixture looks smooth, slowly pour in the chocolate, vanilla extract and salt. Give it one final mix using a rubber spatula to make sure all of the chocolate is incorporated.
8 oz semisweet chocolate, 1 ⅓ cups unsalted butter, ½ teaspoon vanilla extract, ½ teaspoon kosher salt
Assembly
Put some of the buttercream in a piping big with a round tip. Place one cake layer down on to your plate and spread just a bit of the frosting on the cake layer to create a barrier. Then use the piping bag to pipe a barrier around the outer edge of the cake - this is so the filling won't leak out!
Spoon the cooled strawberry filling on top of the cake, using an offset spatula to spread it to the barrier you've created. You may choose to have a thin layer of strawberry filling or a thick one - it's up to you!
Gently place the second layer of cake on top of this and repeat once more.
Place the top cake layer on top and frost the cake with the remaining frosting. I used a Wilton 1M piping tip to create a swirly border!
Notes
My recipes have been developed and tested in metric grams. I highly recommend using a weighing scale and measuring out your ingredients the metric way. Just click on 'METRIC' above the ingredients list to get the metric measurements. I make sure that all of my ingredients are at room temperature before I start the recipe, so pull out all of the ingredients before you start baking.Butter: I use European style butter in my recipes. This will make your frosting much creamier and richer. For the buttercream, you want the butter to be at room temperature - it should be cool to the touch. Salt: If you don’t have kosher salt, halve the amount of salt called for in the recipe. 1 teaspoon kosher salt = ½ teaspoon fine salt. Please note I am using Diamond Crystal salt, not Morton salt which is much saltier. Buttermilk: If you don’t have buttermilk, replace it with an equal amount of sour cream or full fat yoghurt. You can also make your own buttermilk, for every cup of buttermilk add 1 tablespoon of lemon juice to 1 cup of milk and let sit for 5 minutes. Vegetable Oil: Use an unflavoured oil like rapeseed or canola oil if you don’t have vegetable oil. Two Layer Cake: if you prefer to have a two layer cake, use the same amount of ingredients but just bake in 2 8-inch cake pans, bake for 35-40 minutes.Egg Whites: I recommend weighing out the egg whites to make sure you have the correct ratio of egg whites to sugar. Fixing 'broken' buttercream: if your buttercream is having issues it's either because the meringue didn't have enough time to cool down, or the butter you added was too cool. If it's looking soupy, then your meringue is still too warm. Pop the mixing bowl in the fridge for 20 minutes and then keep going! Storage: Keep tightly wrapped in the fridge for up to 5 days. Let it come to room temperature before eating.
Making Ahead: You can make the cake layers ahead of time. Bake them and then once completely cooled, wrap in plastic wrap tightly. Store in the fridge for up to 2 days or in the freezer for up to 2 months. If frozen, allow to defrost and come to room temperature before frosting the cake.