In a large mixing bowl combine all purpose flour or cake flour, brown sugar, cocoa powder, espresso powder, baking powder, baking soda, and kosher salt. Give it a good whisk and set aside.
In a separate bowl, add all of your wet ingredients and whisk together - vegetable oil, melted butter, eggs, sour cream, and vanilla extract. Whisk together for 3 minutes, until pale and fluffy.
Add half of the flour mixture into the wet ingredients and mix together.
Then pour in the hot coffee and mix until combined. Add the remaining flour mixture and mix until just combined.
Fold in a cup of chocolate chips and cover the bowl with plastic wrap. Allow the muffin batter to rest at room temperature for 30 minutes.
Preheat the oven to 350 degrees F. Spray your standard 12-cup muffin pan with oil. Add muffin liners to every other muffin cup and fill with 1/3 cup of batter (fill to the top but don't overflow). Bake for 15-20 minutes, until a toothpick poked in the center of the muffin comes out with just a few crumbs. Allow them to cool in the muffin pan for 5 muffins, then remove and allow them to cool completely on a wire rack.
Add heavy cream to a large bowl. Warm in the microwave for a minute, then add the dark chocolate.
Whisk gently until the chocolate is fully melted. (if the chocolate doesn't melt completely, heat in the microwave in increments of 15 seconds until the chocolate is fully melted). Once the muffins have cooled, dip the tops of the muffins in to the melted chocolate, then top with additional chocolate chips and flaky sea salt.
Notes
Let the muffin batter rest! Don't skip this step, its what makes these muffins super moist and lets them rise sky high in the oven. Totally worth the wait, I promise!
To prevent dry muffins, be sure not to overbake. Bake just until a toothpick comes out with just a few moist crumbs.
For tender chocolate muffins, be sure to mix the batter until it's just combined and there are no visible patches of flour. Don't overmix the batter, this will result in tough dry muffins.
Use room temperature ingredients. This helps the batter comes together smoothly. You'll want your buttermilk, eggs, and sour cream to all be at room temperature. Leave them out for 30 minutes prior to baking for best results.
I recommend baking one pan of muffins at a time. This allows for the tops of the muffins to get nice and high.
If you have a kitchen scale, ditch the measuring cup and measure out the ingredients in grams (exact measurements are provided in the recipe card). This ensures the most consistent bake every time. If you don't have a kitchen scale, fluff up your flour prior to scooping it out to prevent adding too much flour to the batter.
Do not open the oven door while baking! Temperature control is key to getting fluffy muffins. Keep the oven door close until the muffins are done!