In a large bowl and with an electric mixer, beat cake mix, eggs, milk and butter for 2 minutes at medium speed, scraping the sides and bottom a couple of times so everything is incorporated.
Scoop out about ¾ cup of the batter into another bowl. Stir in the cocoa powder.
Grease a 10-inch bundt pan with nonstick cooking spray. Pour in half about ⅔ of the yellow cake batter. Drop spoonfuls of the chocolate mixture evenly on top of the yellow. Pour the remaining yellow cake batter evenly on top. Take a knife and make a few swirls into the batter. *Note - take care not to scrape the metal pan with the knife or it will scratch.
Bake for 50-60 minutes or until a toothpick inserted into the center comes out clean.
Cool in the pan for 10-15 minutes, then turn out onto a wire rack to cool completely. Transfer to a serving platter and dust with powdered sugar, then slice and serve.
How to store leftover cake: Keep the cake tightly wrapped on a platter, or in an airtight container. It should stay fresh and moist for about 3 days. Do not refrigerate or it will dry out.
How to freeze leftover cake: Wrap the whole cake or individual slices tightly with 2 layers of plastic wrap, then place in a freezer bag or container. Freeze for up to 6 months.