Food coloring in gel, powder or liquid form can be used. Instead of the frosting, white chocolate melted in a double boiler can be colored, as long as the food coloring is not liquid.
Chocolate cake: Melt the butter in a double boiler, chop the dark chocolate and add it to the butter. Mix well until all the chocolate is melted, add the cacao powder and continue to mix until the cream is thick but without lumps. Turn off the heat and let cool. While it’s cooling, chop the hazelnuts into large pieces. In a bowl beat the eggs with the sugar, then add the mixture slowly to the chocolate. In another bowl, sift the flour, the baking powder and a pinch of salt, add the chocolate mixture a little at a time while continuing to mix to prevent lumps. Pour the mixture into a rectangular pan, about 20x28cm lined with parchment paper and bake in a preheated oven at 180° for 35 minutes. After the cake has cooled, use a tree-shaped cookie cutter to make the brownies.
Prepare the water frosting: put the powdered sugar with a little boiling water in a bowl, mix vigorously to melt the sugar. The consistency should be sticky and quite dense, if it’s too liquid, add more powdered sugar and if it’s too thick add more water. Add your food coloring of choice and mix vigorously to make the coloring homogeneous.
Decorate freely or with a pastry bag.