Preheat the oven to 350°F. For high-altitude baking, raise the temperature to 375°F. Grease and flour two 9-inch cake pans so the layers release cleanly after baking.

In a medium mixing bowl, sift together the flour, cocoa powder, baking powder, baking soda, and kosher salt. Set the dry mixture aside once it is evenly combined.

Place the softened butter and granulated sugar in a large bowl. Using a mixer on medium-high speed, beat until the mixture becomes pale and fluffy, about 3 to 5 minutes. This stage helps create a lighter cake texture.

Reduce the mixer speed to low. Add the vanilla extract, then incorporate the eggs one at a time, mixing well after each addition. Scrape down the bowl as needed to ensure the batter blends evenly.

With the mixer still on low, add half of the sifted dry ingredients to the butter mixture, followed by half of the milk. Mix just until combined. Repeat with the remaining dry ingredients and the remaining milk, blending only until the batter is smooth. Do not overmix.

Divide the batter evenly between the prepared cake pans. Bake for 20 to 25 minutes, or until a cake tester inserted into the center comes out clean.

Let the cakes rest in their pans for about 10 minutes. Then transfer them carefully to a wire rack and allow them to cool fully before proceeding with frosting or further assembly.