Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans or line them with parchment paper.
In a large mixing bowl, sift together the all-purpose flour, cocoa powder, baking powder, and sugar. Set aside.
In a separate bowl, whisk together the eggs, mayonnaise, vegetable oil, and vanilla extract until well combined.
Slowly add the wet ingredients into the dry ingredients and mix until smooth.
Gradually stir in the boiling water. The batter will be thin, but that's perfectly fine as it ensures a moist cake.
Divide the batter evenly between the prepared cake pans. Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
Let the cakes cool in the pans for about 10 minutes before transferring them to a wire rack to cool completely.
Once the cakes are completely cooled, spread your favorite chocolate frosting over the top and sides of the cakes.
Slice and enjoy your rich, moist, homemade Copycat Portillo’s Chocolate Cake!