Preheat your oven to 350°F/175°C. Grease and line two 8-inch cake pans
In the bowl of a stand mixer with the paddle attachment, mix all of the dry ingredients on slow speed until blended. Add in your butter and mix on low until it looks like damp sand
Add your egg whites one at a time, and then your whole egg. Beat well after each addition, making sure you scrape the sides and bottom of the bowl.
In a small jug, whisk together the vanilla extract, yogurt, lemon juice, lemon zest and milk.
Slowly pour in one-third of the milk mixture into the mixing bowl. With the speed on low, mix together. Be careful as it may spill! Pour in the rest of the milk in two more additions, mixing slowly after each addition.
Beat for 2 minutes. Your batter will be liquidy
Pour the batter into your two pans. Bake for 25-30 minutes. A toothpick inserted into the cake will come out clean. Remove from the oven and let cool for 15 minutes. Take the cake out of the tin and let it cool completely.
Make Filling
In a medium saucepan over medium-low heat, stir together the berries and water. Cook for 15 minutes. The fruit will start to break down.
Whisk together sugar and cornstarch in a small bowl. Add sugar mixture to the juice and turn heat to medium, cook until it comes to a boil - about 3 minutes
Pour into a bowl and let come to room temp. Can be stored for 3 days in the fridge in an airtight container.
Make Frosting
In a stand mixer fitted with the paddle attachment, beat the butter and powdered sugar together on low speed
Increase the speed to medium-high and beat until it becomes thick
Add the lemon juice, lemon zest and salt and beat on medium-high
Turn the mixer to low, and pour in the cream until it becomes creamy and spreadable
Assemble
Place one cake layer down. Scoop out some of your lemon buttercream and use a small offset spatula to spread a layer of frosting down. If you wish you could make a barrier to stop the filling from spreading out. Do this by placing some of the frosting in a piping bag and pipe around the border of the cake.
Scoop out your chilled blueberry filling and use a spoon or offset spatula to smooth out the filling on top of the lemon buttercream. Sprinkle over a small handful of extra fresh bllueberries if you wish! If you didn't make a barrier, then just make sure to leave a half inch border all around the cake so it doesn't seep out.
Place the second layer on top and press down gently. Evenly cover the sides and top of the cake with the rest of the frosting.
Decorate the top of the cake with sprinkles, blueberries or candied flowers.
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 ingredients.
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, like Kerry Gold, in my recipes. This will make your frosting much creamier and richer.
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.
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. How to make candied flowers: Before picking flowers, please make sure they are safe to place on cakes and are edible! I didn't actually eat the flowers but better to be safe. Gently wash the flowers. In a small bowl, whisk an egg white with a few drops of vodka or water. Carefully brush a thin layer of this mixture on to the entire flower, making sure you get the back of the petals. Sprinkle over granulated sugar. Place the flowers on parchment paper and let them dry overnight. The flowers can keep in an airtight container for a few weeks.