Preheat the oven to 180°C (350°F).

In a large bowl, whisk eggs and sugar with an electric mixer until thick and light in color.

Add vanilla and vegetable oil, and keep whisking well.

Add milk and whisk for another minute.

Add flour, baking powder, and salt, and whisk for 2 more minutes until you get a smooth, even batter.

Divide the batter into two parts. Keep one part plain. Add cocoa powder to the second part and mix well until it turns brown.

Grease a baking pan with butter and line the bottom with parchment paper.

Spoon a scoop of plain batter into the center of the pan, then a scoop of cocoa batter on top. Alternate scoops until all batter is used.

Bake for 45 minutes or until the cake is fully cooked.

Let the cake cool to room temperature before serving.