Preheat the oven to 180°c (350°f/mark 4) and line two baking sheets with parchment paper.
Sift the flour, cocoa powder, baking powder and salt into a bowl.
In another bowl, using an electric whisk, cream the sugar and the butter until pale and fluffy. Gradually add the eggs, mixing well to combine.
Add the vanilla extract to the buttermilk and, alternating with the dry ingredients, add to creamed butter mixture, mixing until everything is combined and you are left with a thick batter. Spoon rounded teaspoons of the batter on to the prepared baking sheets, allowing space for them to spread while cooking. Bake in the oven for about 15 minutes or until the tops have puffed up and spring back when lightly pressed.
While the mini pie halves are baking, prepare the filling by beating the butter in a bowl until pale and creamy. Add the vanilla extract and gradually add the icing sugar until the mixture is light and fluffy.
When the mini pie halves are cooked, leave them on the baking sheets for 5 minutes then remove them from the baking sheets with a metal spatula and place on wire rack to cool. Once cooled, spread a rounded teaspoon of the creamy filling on the flat side and sandwhich together with another.