1. Preheat the oven to 180°C (350°F). Lightly grease a square baking pan* (26x26cm) and line it with baking parchment. Set aside. Melt the butter for the brownies and let it cool down slightly. Chop the walnuts and the chocolate and set both aside.

2. Add the eggs and both sugars to a large bowl and mix on high for a few minutes until the mixture is light and fluffy. Add the slightly cooled butter and vanilla extract and mix until well combined. Combine the flour, cocoa powder, baking powder, and salt, then add these dry ingredients to the bowl and mix in. Next, add the chopped walnuts and chocolate and fold in.

3. Transfer the batter to the prepared baking pan and spread it evenly with an offset spatula. Bake the brownies in the preheated oven for about 35-40 minutes – or until a toothpick inserted into the brownies comes out almost clean – some moist crumbs are absolutely fine. Take the brownies out of the oven and let them cool down inside the pan.

4. For the buttercream, add butter, vanilla extract, and salt to a large bowl and mix on high for a few minutes until light and fluffy. Add the sweetened condensed milk and mix another 2-3 minutes. The mixture will initially seem quite runny, but the longer you mix, the thicker and firmer the buttercream will become. Divide the buttercream into three batches – a small one should stay without food coloring (for the white parts of the Santa hats) and the rest should be colored red and green – roughly half of the remaining buttercream in each color.

5. Remove the brownie from the pan and cut it into three strips. Then cut each of these strips into five triangles, with the sides slightly longer than the base. There will be some scraps left over at the beginning and end of the strips, which can, of course, be eaten. Quality control, right?! ;)

6. Fill each colored buttercream batch into a piping bag fitted with a round or star piping tip (the star piping tip is for the green buttercream) and then decorate the brownie triangles. If you like, you can sprinkle the Christmas trees with some additional sugar pearls (optional).