How to brown butter: Slice the butter stick into a few smaller pieces. Place into a skillet over medium heat and whisk frequently. Butter will melt gradually and then foam up a bit. Watch carefully as brown specks begin to form at the bottom of the pan, a nutty smell should emerge. Remove from heat and pour into a bowl to stop the butter from cooking further and burning.
Preheat oven to 425º F. Line a muffin tin with liners.
In a large bowl, whisk together the flour, baking powder, baking soda, salt, sugar, and espresso powder.
In another bowl, whisk together the browned butter, eggs, milk, and vanilla extract. Make a well in the dry ingredients. Add wet mixture into the well and stir until thoroughly mixed.
Add 1 1/2 cups chocolate chips and fold in gently until just evenly incorporated.
Scoop even amounts of batter into the prepared muffin tin. Sprinkle the additional 1/4 cup of chocolate chips and coarse sugar to taste across the top of the muffins.
Bake muffins for 7 minutes at 425°F. Keep oven closed, reduce temperature to 375°F and bake for 8 more minutes or until the muffins are golden brown and an inserted toothpick comes out clean.
Muffins are delicious warm from the oven or rewarmed in the microwave! Store extras in an airtight container for up to a week.