Preheat oven to 350 degrees F. Grease and flour a 10 inch bundt pan or tube pan.
Whisk together the eggs; sugar and oil in a large mixing bowl until pale and frothy. Stir in the vanilla extract.
Sift the flour, baking powder and salt over the bowl with the wet ingredients. Use a spatula or wooden spoon to combine all the ingredients.
Pour about 2/3 of the batter in the prepared pan.
Stir cocoa powder and milk into the remaining batter in the mixing bowl.
Pour the chocolate batter over the vanilla batter in the pan. Use a skewer or butter knife to swirl the batters together.
Bake for 40-45 minutes until a toothpick inserted in the center of the cake comes out clean.
Transfer to a wire rack and let sit for 15 minutes before unmolding the cake onto the wire rack.
Let cool completely before serving with a dusting of powdered sugar (optional).
Notes
Substitutions and VariationsThe vegetable oil may be replaced with olive oil, if you prefer.StorageKeep leftovers, well wrapped, at room temperature for 4-5 days.To FreezeWrap well in plastic wrap and freeze for up to 1 month. Defrost at room temperature.