First, beat the eggs until whitened. Without stopping beating, subsequently add the sugar, oil, milk, the flour mixed with the baking powder and the orange zest.
Set aside a small part of the cake mixture, mix it with 1 tbsp cocoa. Alternate between the 2 mixtures in an oiled and floured form. Bake about 1 hour in a 356°F (180 °C) oven.