Make the muffins: Heat the oven to 425 degrees with a rack in the center. Line a standard muffin tin with paper liners, then lightly coat the whole pan (including the top and the muffin cups), with nonstick cooking spray.

Whisk the flour, both sugars, baking powder, baking soda and salt in a medium bowl.

Combine the coffee, butter and cocoa powder in a large bowl and let sit for a minute. The heat of the coffee will start to melt the butter. Whisk until very smooth, then whisk in the eggs until smooth. Add the dry ingredients and stir with the whisk until no pockets of flour remain.

Divide the batter, then ¾ cup/108 grams of the chocolate chunks, evenly among the prepared muffin cups. Batter will be thin, so consider using a scoop or a measuring cup. The chunks will sink, and the batter will reach the tops.

Bake until crackly on top and a toothpick inserted in the center of one comes out with a few crumbs, 15 to 18 minutes. Be sure to not overbake to keep these moist. The muffins will continue to bake as they cool in the pan.

Immediately after the muffins come out of the oven, gently press the remaining ¾ cup/108 grams chocolate chunks on the tops. They’ll melt a bit and stick. You can enjoy the muffins on their own, or you can cool them a bit to fill while they’re still warm.

If you’d like to make the optional filling, prepare it while the muffins bake: In a small saucepan, combine the sugar, cream, cocoa powder, butter and salt. Stir over medium heat until smooth, bubbling and glossy, 1 to 2 minutes. Remove from the heat and cool until warm, stirring occasionally. Transfer the filling to a piping bag or resealable plastic bag.

Stick a paring knife into the center of each warm muffin almost all the way to the bottom. Snip a small hole in the tip of the piping bag, insert it in a muffin slit and squeeze in the warm filling until it comes out the top. Repeat with the remaining filling and muffins. The muffins are best when eaten warm, but still taste good at room temperature, too.