Preheat the oven to 350° Fahrenheit or 180° Celsius.

Grease the Bundt Pan with a thin coat of butter.

Using a saucepan, melt the butter over a low heat.

Gently swirl the butter in the saucepan as it melts to make sure it doesn't burn.

Set the melted butter aside to cool off.

Using two mixing bowls, separate the egg yolks and egg whites.

Beat the egg whites with a hand mixer until stiff and fluffy.

In a separate bowl, combine the egg yolks with powdered sugar, brown sugar, vanilla, and salt, and using a hand mixer equipped with a whisk, beat them thoroughly until the mixture obtains a creamy and fluffy texture.

Add the egg whites to the mixed egg yolks and gently stir them together.

In a large measuring cup, combine pastry flour, potato flour, and baking powder, and mix them thoroughly together with a spoon.

Slowly add the flour to the combined eggs, and mix them gently together.

Continue mixing gently as you slowly pour the butter. Continue mixing all the ingredients together until combined into a batter that's uniform in texture.

Pour ¾ of the mixed batter into the greased Bundt cake pan.

Add cocoa powder to the remaining batter and mix it well together with a spatula.

Pour the cocoa batter evenly on top of the vanilla batter.

Put it in the oven and bake the Bundt cake for 40 - 45 minutes.

Remove your Bundt cake from the oven and let it cool for 10 - 15 minutes.

Slowly turn the Bundt pan with the cake onto a large plate, and lift the form as you rest the Bundt Cake on the plate.

Sprinkle some powdered sugar on top and enjoy!