|1 tsp||baking powder|
|180 g||egg whites|
|500 g||unsalted butter (room temperature)|
|confectioner’s sugar (for dusting)|
|blackberry (for garnish)|
Preheat oven to 180°C/355°F. Add eggs and some of the sugar to the bowl of a stand mixer and beat until creamy. Add yogurt and keep beating. Add baking powder to flour and add the mixture to the stand mixer bit by bit.
Spread batter on a parchment-lined baking sheet. Bake at 180°C/355°F for approx. 10 min. In the meantime, dust a clean kitchen towel with confectioner’s sugar. Once the cake is done, remove from the oven and invert it onto the kitchen towel. Remove parchment paper and cover the cake with a second moistened kitchen towel. Carefully roll cake and towels and let cool.
Add blackberries to a liquid measuring cup and purée until smooth. Pass through a fine sieve set over a saucepan. Add remaining sugar and heat the mixture over medium heat. In the meantime, beat egg whites and some salt in the bowl of a stand mixer until stiff peaks form. Remove the blackberry syrup from heat as soon as it reaches 115°C/240°F. While beating the egg whites, slowly add the blackberry syrup. Keep beating until the mixture is at room temperature, then add diced butter (bit by bit) and keep beating to form a buttercream.
Cut cooled sponge cake into 3 equal strips. Spread each strip with some of the blackberry buttercream. Carefully roll up the first strip, then position the seam of the first roll at the beginning of the next one and keep rolling up. Repeat with the third stripe, then add the cake to a platter. Spread remaining buttercream over the entire cake. Before slicing, let cake rest in the fridge for at least 1 hr. Garnish with fresh blackberries and enjoy!