Place the butter and chocolate in a heatproof bowl over a saucepan of just simmering water, stirring occasionally, until melted.
Sift flour, baking powder and cocoa together and add to melted chocolate mixture, along with sugar, vanilla and eggs. Mix until well combined. Cover mixture and refrigerate for 10 minutes.
Meanwhile, preheat oven to 180°C/160°C fan forced. Line 2 baking trays with baking paper. Roll dough into small walnut sized balls and place on prepared trays, leaving room for spreading. You should have around 30 balls of dough.
Flatten each ball slightly with your hand and bake 12 minutes. The cookies will still feel soft but will firm up as they cool. Transfer trays to a wire rack and let cookies cool on trays for 2 minutes, then use an egg slice to carefully transfer cookies to a cooling rack to cool completely.
To make filling, place all ingredients in the bowl of an electric mixer and beat on a low speed until combined. Increase speed to medium and beat for for 2 minutes longer or until light and fluffy. Pipe or spread filling evenly over 15 of the cooled cookies. Sandwich another cookie on top and press together gently. Refrigerate for 30 minutes.