Cream butter and sugar until fluffy.

Add egg white, vanilla extract and sour cream and mix.

Combine flour with baking powder and cocoa and add to the butter mixture. Mix until just incorporated.

Wrap the dough in plastic cling film and refrigerate for at least 2 hours.

Roll out the dough to a 1/5 inch (5 mm) thickness. Cut out cookies using a Christmas star shaped cookie cutter.

Place cookies on a baking sheet lined with parchment paper and bake for about 5-8 minutes in 375 F (175 C). Let cookies cool on the cookie sheet.

Spread a layer of nutella on a cookie. Add another cookie on top to create a sandwich. Repeat with all cookies.

Dip cookie sandwiches in melted chocolate and place on a wire rack. Sprinkle with golden sprinkles.