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 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 pans, it's about 650g per cake tin.
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.
Swiss Meringue Buttercream
Fill a small saucepan with about an inch of water, bring to a simmer
In a stand mixer bowl, stir the egg whites and sugar until combined. Place the bowl over the saucepan. Make sure you're constantly whisking the sugar mixture until the sugar dissolves. Feel the mixture between your fingers to check if there are granules
Once it is smooth, remove the bowl from heat and place it in your stand mixer, with a whisk attachment. Whisk on medium high until you have stiff, glossy peaks, about 10-12 minutes. Touch the bowl, at this point it should have cooled down.
Reduce speed to low and add the butter, a tablespoon at a time, and beat until combined. Add the vanilla and salt
Remove about ½ cup of frosting and set aside. Pour the jam into the rest of the frosting and mix to combine. Use immediately
Assemble
Take about ½ a cup of blueberry frosting in a piping bag and pipe a dam around the edge of one of the cakes. Scoop out the vanilla frosting and use an offset spatula to spread the frosting evenly inside the dam. Take your jam and use a spoon to spread the jam on top of the vanilla frosting.
Gently press down your second layer on top. Crumb coat the sides and top of cake by spreading a thin layer of frosting, then chill the cake in your fridge for about 30 minutes.
Take the cake out and frost it with the rest of the buttercream.