You'll need flour, baking powder, salt, butter, sugar, milk, yogurt, oil, vanilla, lemon zest, and egg whites.
Make the batter
Combine the dry ingredients, then mix in the butter until it's pea-sized. Mix in the wet ingredients. Lastly, fold in the whipped egg whites.
Divide the batter between three 6inch cake pans and bake at 350 degrees F for about 30 minutes.
Cool on wire rack
Turn the cakes out onto a wire rack to cool completely.
Make the buttercream
Make the german buttercream by beating butter and cream cheese into the pastry cream until smooth.
Pipe a dam of buttercream around the edge of the first cake layer. Fill in the center with blueberry jam. Repeat with the next layer and top with the final cake.
Spread a thin layer of buttercream all over the cake for the crumb coat, then chill in the fridge for 20 minutes. Then, decorate the cake as desired.
Cut the cake and serve!