Whisk eggs and sugar until fluffy.
Melt the butter and add it to the egg mixture along with the remaining ingredients.
Gently whisk everything together.
Pour into a greased baking pan or a baking tray lined with parchment paper and bake for about 40 minutes at 175 degrees Celsius (347 degrees Fahrenheit) (fan-forced).
Once the layers have cooled, cut them into the desired letter or number shapes.
You can split the letter/number into two if you want 2 layers and thus 2 mousse layers in your cake.
But you can also start with just one layer, using the layer as it is.
Put your mousse in a piping bag with a star nozzle, decorate the cake, and top with goodies.