Place the butter and caster sugar into your Thermomix bowl and mix for 30 seconds, speed 3.
Scrape down the sides and mix for a further 20 seconds on speed 3 before increasing to speed 5 for 10 seconds.
Add the egg and vanilla essence and mix for 10 seconds, speed 4.
Add the flour, cocoa and baking powder to your Thermomix bowl and mix for 10 seconds on speed 4 - you want a dough to start coming together. Mix for a further 5 seconds if needed.
Pour the dough onto a Thermomat (or a floured bench) and gently bring it together with your hands to form a ball. Lightly press down on the dough to flatten into a disc shape before wrapping the dough in cling wrap and placing into the fridge to firm for one hour.
Preheat your oven to 190 degrees and line two trays with baking paper
Divide the cooled dough in half and roll one half between two sheets of baking paper until it’s approximately 5mm thick. Use an 8 cm round cookie cutter to cut out the biscuits.
Place the biscuits onto the prepared baking trays and bake for 10 - 12 minutes or until slightly firm to the touch. Remove the biscuits from the oven and let them cool on the trays for 5 minutes before carefully transferring to a wire rack to cool completely.
Break the milk chocolate into pieces and place into a microwave safe bowl. Cook on a medium heat for 30 seconds before stirring and cooking for a further 30 seconds or until the chocolate has melted. Dip each cookie into the melted chocolate and decorate with sprinkles.
I used an 8cm round cutter for this recipe, if your cutter is larger or smaller you will need to adjust the cooking time accordingly.