Preheat the Oven: Start by preheating your oven to 350°F (175°C). This ensures an even baking temperature for your cake.
Prepare Your Baking Pan: Grease and flour a 9x13-inch baking pan. This helps the cake to easily release once baked.
Mix the Wet Ingredients: In a mixing bowl, beat the eggs, sugar, and oil together until the mixture is light and fluffy. This should take about 3-5 minutes!
Add Milk and Flour: Gradually mix in the milk and then the flour until combined. Ensure there are no lumps!
Incorporate Baking Powder: Add the baking powder and mix until just combined.
Divide the Batter: Pour half the batter into a separate bowl. To one half, add the chocolate milk and mix until smooth.
Layer the Batter: In the prepared pan, pour in half of the vanilla batter, followed by the chocolate batter. Top it with the remaining vanilla batter.
Create Swirls (Optional): Use a knife to gently swirl the batters together for a marbled effect.
Bake: Place your cake in the preheated oven and bake for approximately 45 minutes, or until a toothpick inserted in the center comes out clean.
Cool and Serve: Once baked, remove from the oven and let it cool in the pan for 10 minutes before transferring it to a wire rack to cool completely. Slice and serve!