These were originally an American recipe that I've anglicised. They were made famous here in Ireland by Sex in the City and are slowly taking off here, so much so that I've even seen them for sale in my local SuperValu!
260g Plain flour
50g Cocoa powder
1 1/4 tsp Bicarbonate soda
250ml Buttermilk (well-shaken)
1 teaspoon vanilla
130g Butter, softened
140g Dark brown sugar (sometimes I use light brown sugar instead)
1 large egg + 1 egg yolk
250g icing sugar
2 egg whites
1tsp Vanilla extract
A pinch of salt
Preheat oven to 180°C and line two large baking sheets with baking paper.
Beat together butter and brown sugar in a large bowl with an electric mixer at medium-high speed until pale and fluffy, about 3 minutes in a standing mixer or 5 minutes with a handheld, then add egg, beating until combined well. Mix together flour, cocoa, baking soda, and salt in a bowl until combined. Stir together buttermilk and vanilla in a small bowl.
Reduce speed to low and alternately mix in flour mixture and buttermilk in batches, beginning and ending with flour, scraping down side of bowl occasionally, and mixing until smooth.
Bake in upper and lower thirds of oven, switching position of sheets halfway through baking, until tops are puffed and cakes spring back when touched, 11 to 13 minutes.
Transfer with a metal spatula to a rack or more baking paper to cool completely.
Make filling: Beat your egg whites until they form stiff peaks.
Spoon the whites out of the bowl and put in the butter. Beat until butter is nice and creamy, then add the icing sugar bit by bit until it looks well blended. Then add your salt, vanilla and egg white mixture and beat until it's all nice and incorporated and fluffy.
Drop rounded spoonfuls onto your little cakes and top with a second cake ( I piped because I wanted the cakes to look fancy for the cover shot and to impress all my lovely readers!!)