Makes 1 cake (using a 24cm springform tin)
Cream the butter and sugar until white and airy. Stir in the eggs one at a time. Divide the mixture between two separate bowls.
Mix together half of the baking powder (2 tsp), salt and flour (250 g) and sieve into the mixture in one of the bowls, alternating with 200 ml of the milk and the vanilla extract. Add a tablespoon of cold water.
Heat up the rest of the milk (250 ml) and add the grated brunost. Stir until the brunost has melted and then allow to cool slightly. Mix together the rest of the baking powder (2 tsp), salt and flour (250 g) and sieve into the second bowl together with the brunost and milk mixture. Add a tablespoon of cold water.
Grease a 24cm springform tin. Alternately pour the brunost mixture and the vanilla mixture into the centre of the tin. This will create a marbled pattern.
Cook on the bottom shelf at 180 ºC for 50-60 mins. Allow the cake to cool.
Mix half of the butter (50 g) with half of the icing sugar (100 g), a splash of milk and the vanilla extract. Stir until you have an airy butter frosting. Use this to top half of the cake.
Melt the other half of the butter (50 g) and add the brunost. Stir until the brunost has melted. Add the icing sugar and about 2 tablespoons of milk. Mix together and use to top the other half of the cake. Decorate with brunost.