Heat the oven to 350°F (175°C). Line a 9 x 13-inch metal baking pan with parchment, leaving enough overhang to lift out the brownies, then lightly grease the parchment.

Whisk the melted butter and granulated sugar together in a large bowl. Add the room-temperature eggs and vanilla extract, then whisk until the mixture looks smooth and well blended.

In a separate bowl, sift or whisk together the flour, cocoa powder, instant espresso powder, and fine salt. This distributes the espresso and cocoa evenly through the batter.

Fold the dry ingredients into the butter mixture just until combined. Stir in the semisweet chocolate chips or chopped chocolate, taking care not to overmix.

Spread the batter evenly in the prepared pan. Bake for 25–30 minutes, until the edges are set and a toothpick inserted in the center comes out with moist crumbs rather than looking completely clean.

Cool the brownies completely in the pan before adding the ganache. For the ganache, place the chocolate in a heatproof bowl, heat the cream until steaming but not boiling, pour it over the chocolate, and let it stand for 2–3 minutes.

Stir the ganache slowly from the center outward until smooth. Stir in the Kahlúa and optional butter, spread it over the cooled brownies, and refrigerate for 30–60 minutes before cutting.