Preheat to 425°F (218°C). Grease a 12-cup muffin tin or line with papers. For jumbo muffins, prepare a 6-cup jumbo tin.
Measure accurately: Spoon and level flour and cocoa. Sift cocoa to prevent bitter lumps.
Use room-temp eggs and buttermilk for easier blending and better rise.
Whisk dry: In a large bowl, whisk flour, cocoa, granulated sugar, brown sugar, baking powder, baking soda, salt, and espresso powder until evenly combined.
Whisk wet: In a separate bowl, whisk buttermilk, oil, eggs, and vanilla until smooth.
Combine: Pour wet into dry. Stir with a spatula just until streaky. Do not fully mix yet.
Add hot water: Stream in hot water and gently fold until the batter looks thick, glossy, and slightly lumpy. Small lumps are fine.
Fold in chips: Stir in most of the chocolate chips, holding back a small handful for the tops.
Fill generously: Fill each cup almost to the top (about 3/4 to heaping full). Sprinkle reserved chips on top.
Start hot: Bake at 425°F for 5–7 minutes to boost oven spring.
Reduce heat: Without opening the door, reduce to 350°F (177°C) and continue baking:
Standard muffins: 13–16 minutes more
Jumbo muffins: 18–22 minutes more
Check doneness: A tester should come out with a few moist crumbs, not wet batter. Tops should look set and slightly glossy.
Cool properly: Rest in the pan 5 minutes, then transfer to a rack. This keeps bottoms from steaming.
Don’t overmix: Overworking gluten makes tough muffins and flat tops.
Thick batter wins: The batter should mound in the scoop. Add 1–2 tablespoons flour if it looks runny.
Use a scoop: A large cookie scoop gives uniform size and even baking.
Rest the batter (optional): A 20-minute rest lets flour hydrate and improves rise.
