1. Get prepping! Preheat your oven to 350 degrees Fahrenheit. Butter your cake pan(s), or, if you’re setting down Gefen Parchment Paper, lightly butter the paper too.
2. Break out your stand or hand mixer with a paddle attachment. On high speed, whip the butter for one minute.
3. Scrape down the mixing bowl with a silicone spatula, then add in the two cups of sugar. Beat the sugar and butter mixture for two minutes on high speed. Repeat your spatula move.
4. Turn the mixer down to medium-low speed; add in vanilla. Beat in the eggs one at a time.
5. Before you add in your dry ingredients, scrape down the bowl once more.
6. In a separate bowl, combine your dry ingredients: flour, cocoa powder, hot cocoa mix, baking powder, and sea salt.
7. With the mixer on low speed, incorporate half of the flour mixture. Once it’s mostly incorporated, slowly pour in half of the milk. Add in the remaining flour mixture and milk and stop the mixer as soon as the batter is smooth. (Overmixing is not your friend!)
8. Pour batter into your cake pan(s). Bake at 350 degres Fahrenheit until the middle of the cake passes the fingerprint test—in other words, a done cake will be springy when you press a finger to the center of the cake. This will be approximately 30 to 35 minutes for a half-sheet pan, 35 to 40 minutes for the round pans, and 60 to 70 minutes for a Bundt pan.
9. Let cool completely.