Preheat the oven to 350 F and place an oven rack in the middle of the oven.
Sift together the flour, cocoa powder, baking powder, and corn starch into a medium bowl.
In the bowl of a stand mixer combine the eggs, sugar, vanilla, espresso powder, and salt. Use the whisk attachment and whip on high for 3 minutes until light and fluffy.
In a large measuring cup, stir together the oil and milk. Once the eggs are fully beaten, turn the mixer to medium low and slowly stream the liquids down the side of the bowl into the egg mix.
Stop the mixer and dump in the dry ingredients. Turn the mixer back on to medium low and whisk again until just combined.
Liberally spray an a 9" cake pan or springform pan with baking spray (one that includes flour) or line with a circle of parchment paper, and pour the batter in.
Place in the preheated oven for 25 minutes. Rotate the cake 180 degrees and bake for another 20 minutes until a toothpick inserted in the center comes out clean.
Once baked, remove the cake from the oven and place the pan on a wire rack to cool for 10 minutes. Run a knife along the edge of the cake to loosen it from the edges of the pan.
Remove the cake from the pan by tipping it out or releasing the springform.
Lift the cake pan off of the cake and place on a wire rack to cool fully.
Once cooled, wrap the cake with plastic or an airtight container and place it in the fridge.