Preheat the oven to 150 degrees and keep the temperature for the cooking time. Whisk together the eggs and sugar. Add the chopped pistachios, the melted and cold butter, the flour and the baking powder. Pour the batter into a greased and floured baking-pan, jump in a hot oven for about 45 minutes. Once cooked, leave the cake in the damped-down oven 5 minutes yet. When is cool, spread the pistachio cream on the top of the cake (you can stuff inside) and garnish with the chopped pistachios.