Cream the butter and sugar until fluffy and light using a standing mixer or hand mixer.

Add the egg, vanilla and salt and make sure they get mixed in properly and scrape down the bowl to help.

Sift in the flour and cocoa powder, then combine.

Move the dough onto some plastic wrap and roll it into a 3.5 cm log. Wrap it with the cling film and let it rest in the fridge overnight.

The next day, when you are ready to bake, turn on the oven to 160°C and let it warm up. 

Prepare a baking sheet with some parchment paper, then slice the biscuit log into 1cm thick slices and place them on the baking sheet.

Bake for 15 to 20 minutes, then let it cool down completely.

Once the chocolate biscuits are fully cool, melt the white chocolate and, using a teaspoon, decorate the top of each biscuit, then place the sprinkles.

Let the white chocolate solidify completely before serving.