Take the ingredients out ahead of time so they're at room temperature. Preheat the oven to 180°C.

Beat the butter with a mixer for a minute, gradually adding the powdered sugar.

Continuing to beat, add the vanillin, salt and egg, mixing until smooth.

Add the dry ingredients - flour, cornstarch and baking powder - and knead the dough.

Pinch off pieces of dough and roll into small balls the size of a walnut, placing them on a parchment-lined baking sheet.

Using a 2cm round piping tip, press a dimple into the center of each ball to form the mushroom stem. Grease the tip with butter first, then dip it in cocoa powder before each following imprint.

Bake the cookies in the preheated oven for 15-20 minutes.