Set a rack in the center of the oven and preheat to 325°F. Line 2 baking sheets with parchment paper.
In a large bowl, whisk the melted butter with the cocoa powder and espresso powder until smooth and glossy. Let sit 2 minutes to slightly cool and bloom the cocoa.
Whisk in the eggs, egg yolk, vanilla, and sea salt until the mixture is uniform and shiny.
Switch to a sturdy spatula and add the cake mix in two additions, folding just until no dry streaks remain. The dough will be thick.
Fold in most of the chopped Reese’s, reserving a small handful for topping. Cover and chill the dough for 10 minutes to firm slightly.
Scoop heaping 3-tablespoon portions (about 22 cookies total) onto prepared sheets, spacing about 2 inches apart. Gently press each mound to about 3/4 inch thick for even baking.
Bake one sheet at a time for 12–14 minutes, rotating the pan halfway through. The edges should look set while the centers remain soft and slightly underdone.
Immediately press a few reserved Reese’s pieces onto the tops. Cool on the pan for 5 minutes, then transfer to a rack to finish cooling. Repeat with remaining dough.