Preheat the oven to 180 ° C and place 12 paper cases in the wells of a muffin tin. Mix the cocoa powder in a small bowl with a little warm water to a paste.
Beat the butter in a pastry bowl until smooth, then add the cocoa paste, sugar, eggs, milk and the flour mixed with baking powder. Mix everything into a creamy batter. Pour the batter into the molds with a teaspoon and bake for about 15 minutes, until the cupcakes have risen and are still soft when pressed with a finger.
Remove the tartlets from the baking tray and place on a wire rack to cool. Mix the mascarpone with the raspberries until creamy and sweeten with a little icing sugar.
After cooling, carefully cut off the top layer of the cupcakes, spread some raspberry cream on each tart and put the cut-off lid back on top. Finally dust the cupcakes with powdered sugar.