Preheat oven to 180ºC (355ºF). Process butter, icing sugar, flour, cornflour, cocoa and egg yolk in a food processor until a soft dough forms. Wrap in plastic wrap and refrigerate for 30 minutes. Roll dough between non-stick baking paper to 3mm thick. Using an oval-shaped cookie cutter, cut out shapes from the pastry and place on baking trays lined with non-stick baking paper. Bake for 8–10 minutes or until the bases are lightly cooked. Cool on trays.
To make the chocolate filling, place the chocolate and cream in a small saucepan over low heat and stir until melted and glossy. Refrigerate until cool. Spread half the biscuits with the filling, then sandwich with the remaining biscuits. Makes 22 filled biscuits.
To decorate biscuits, drizzle with melted white chocolate.
* To check if the cookies are done, look at the bottom not the top. If you wait until the top looks right you risk overcooking them. To check, lift a cookie with a spatula so you can see underneath. The bottom should be a deep brown. Cool on trays as the cookies will continue to cook after you’ve removed them from the oven.