Ideally using a stand mixer with the paddle attachment, mix the butter, caster sugar and light brown soft sugar together until fluffy and paler in colour. This will take 3-5 minutes in an electric mixer
Add the egg and vanilla extract and mix in
Add the plain flour, cocoa powder, baking powder, bicarbonate of soda and salt and mix them in until just combined
Then mix in the chopped Kinder chocolate briefly, just to evenly distribute it
Divide the dough into 12 portions, I weighed the dough and used 70g per cookie
Roll each portion into a ball using the palms of your hands, then flatten the dough out into your hand
Place a teaspoon (around 7-10g) of the Kinderella spread into the centre of the flattened out piece of dough
Fold the dough around the Kinderella spread and pinch the dough together to seal the Kinderella spread inside the dough. Roll it back into a ball shape
Do the same with all of the dough until it is used up. Then put all of them into the freezer for 30 minutes. You can either place the balls of filled cookie dough onto a lined baking tray, or into a tupperware container lined with baking paper
Meanwhile, pre-heat your oven to 180C Fan/200C/400F/Gas Mark 6
After the 30 minutes is up and your oven is pre-heated, place the frozen dough balls onto lined baking trays, leaving space (at least 2") between them to spread. If you don't have enough baking trays, keep the dough in the freezer and bake the cookies in batches
Bake for 12 minutes, remove from the oven and immediately place two Kinder Bueno pieces on top of each cookie, gently pressing them on
Leave the cookies to cool for at least 10 minutes on the baking trays before moving to a cooling rack
Enjoy warm, or leave to cool fully. Store leftovers in an airtight container and eat within 3 days