Beat the butter and sugar until light and fluffy. Beat in the egg yolk and then beat in the vanilla.
In a separate bowl, sift the flour, cocoa powder, baking powder and salt. Add this to the butter and stir just until blended. Divide this into 2 discs, wrap and chill until firm, at least an hour.
Preheat the oven to 175°C and line 2 baking trays with parchment paper. Give the dough one or two kneads on a floured work surface to soften it (this will prevent the dough from cracking when rolling) and then roll it out to just less than 1/4-inch thick. Cut out cookies using a 2-inch cookie cutter and lift onto the baking trays. Any scraps can be re-rolled, and if the dough gets too soft when rolling, simply chill it for 15 minutes before rolling again.
Bake the biscuits for about 8 minutes, until they loose any shine to them. Allow the biscuits to cool on the tray before removing to fill.
Roll out the fondant, using icing sugar for rolling (not flour) to just over 1/8-inch thickness and cut 2-inch circles using the same cookie cutter as for the biscuits. Brush one side of the fondant with a little water and press this onto the bottom of one cookie. Brush the other side of the fondant with a little water and place the bottom of a second biscuit, pressing gently to secure the sandwich. Repeat with the remaining biscuits, re-rolling the fondant if needed.
Store the biscuits in an airtight container for up to 3 days.