Preheat the Oven and Prepare Your Baking Tray - Preheat your oven to 350 degrees Fahrenheit. Grease your baking tray with cooking spray. If desired, sprinkle bread crumbs or ground nuts on the tray to prevent any sticking.
Combine Egg Yolks and Sugar - Using an electric mixer, combine the egg yolks and sugar in a large mixing bowl. Beat the mixture for about one minute, until smooth.
Add the Milk, Oil, and Rum - Slowly add the milk, oil, and rum into the sugar mixture. Mix until just combined.
Whisk Egg Whites - In a medium bowl, whisk the egg whites with an electric mixer. Beat until the egg whites are very stiff.
Mix in the Flour and Baking Powder - Add the egg whites to the top of the sugar mixture. Then, add the flour and baking powder to the top of the egg whites. Use a whisk to slowly stir the mixture together.
Add the Chocolate Powder - To half of the cake batter, add the chocolate powder. Pour the regular batter into the cake pan. Then, pour the chocolate batter into the cake pan.
Bake the Cake - Bake the cake for 45-60 minutes. After 45 minutes, use a toothpick or metal skewer to check if the cake is done. If it comes out clean, the cake is done, if there are crumbs or batter on it, continue to bake.
Let the cake cool in the pan for 10 minutes before placing it on a wire rack to cool. Serve the cake once cooled.