Preheat oven to 350 deg F (Gas mark 4 or 180 deg C).Line a 13 x 9 in (33 x 23 cm) cake tin with grease proof or other non-stick paper and grease the tin. Melt the butter. Beat eggs with sugar, and add vanilla, flour, cocoa, baking powder, salt (optional) and melted butter.Add chopped nuts.Bake at 350 degrees until a wooden pick inserted in center comes out clean, approximately 20-30 minutes. Cool the cake. Dust with powdered sugar or glace with your favorite chocolate frosting.