Preheat oven to 175°C (350°F). Line 2 baking trays with parchment.
In a bowl, whisk flour, cocoa powder, baking powder, and salt.
In a large bowl or your stand mixer, cream butter and sugar until light and fluffy.
Mix in egg yolk and vanilla.
Add dry ingredients and stir until dough comes together.
Roll into 1-inch balls, place on trays, and press a thumbprint into each.
Bake 10–12 minutes until just set. Re-press centres if needed. Cool completely.
Heat cream until steaming, pour over chocolate, let sit for two minutes, then stir smooth.
Spoon or pipe ganache into cookie centres. Allow to set before serving. Add chocolate sprinkles or any other toppings. I grated a little bit more chocolate over the cookies but feel free to embellish them as you like!