Gently melt butter over a low heat in a small saucepan.

Whisk together butter and caster sugar in a medium sized bowl.

Break chocolate into small pieces and gently melt in a heatproof bowl suspended over a pan of barely simmering water.

Whisk vanilla and melted chocolate into the butter/sugar mixture.

Whisk in the egg until well combined.

Sift together flour, cocoa powder, salt and bicarbonate of soda and stir into the chocolate mixture until well combined.

Cover and refrigerate for two hours, or overnight to firm up.

Bring dough to room temperature.

Preheat oven to 160 C

Cut dough into quarters and roll each quarter to a 1/4 inch thickness.

Using a 2.5 inch circle cookie cutter cut out shapes and place onto a piece of baking paper on a baking tray.

Using a glass with a diameter 1/2 inch smaller than the cookie cutter, press a small circle onto the cut cookie dough.

Using a drinking straw, cut four button holes out of the centre of the cookie.

Chill cookies in the refrigerator for 10 minutes before baking for 20 minutes.. Transfer to a wire rack to cool completely before icing and assembling.

To make the filling combine the butter, vegetable shortening, icing sugar, vanilla and milk in a food processor and process until light and creamy.

Place 1 tbsp of filling onto the bottom of one cookie and sandwich together with another cookie. Press gently in the centre so that some of the filling shows through the button holes.

Repeat with the remaining cookies. Cookies can be refrigerated to harden the filling, if desired.