This is probably the most common chocolate cake in Sweden these days. It´s super easy to do:

150 gr butter
3 dl sugar
1 dl cocoa powder
1 teaspoon vanilla sugar
2 eggs
2 dl wheat flower

Preheat the oven to 175 º C.
Melt the butter in a pan that holds 2 lit. Turn off the heat. Add sugar, cocoa and vanilla sugar. Stir. Add the eggs, one at the time. Add flower. Stir until you have an even batter.
Pour the batter into a prevously buttered and flowered baking dish. Bake in the bottom of the oven for 20 minutes - not more! Cool cake and put it in the refrigerator for at least 1 hour. Served chilled with whipped cream or vanilla icecream.