Preheat the oven to 350 degrees F (180 C) and grease a 10 to 12 cup bundt cake pan.
Add the eggs, sugar, and oil into a stand mixer and beat until creamy for about 5 minutes. You can also use a hand mixer.
Add the milk, vanilla extract, and salt and whisk to combine.
Then sift the flour and baking powder, and gently combine.
Divide the batter into two different bowls, sift the cocoa powder into one of them and mix to combine.
Pour a layer of vanilla batter into the cake pan, then pour a layer of the chocolate batter on top. Repeat this process until you've used all the batter. The batter will be thick and have a zig-zag form when you add it on top of each layer, don't worry about making it look perfect. That's exactly what is going to give the marble-like appearance.
Bake for about 40 to 45 minutes or until a toothpick comes out clean.
When the cake has cooled down, make the chocolate glaze. Melt the chocolate with heavy cream and whisk until smooth. Wait until the ganache has cooled down a bit, achieved a creamier consistency, and then pour it over the cake.