Lightly grease an 8- or 9-inch square baking pan with nonstick spray or butter. Line it with parchment paper, pressing it into the corners and leaving a couple of inches of overhang on two sides to create a sling. Set aside.

In a medium, heavy-bottomed saucepan, add the semi-sweet chocolate chips, sweetened condensed milk, and butter. Set the pan over low heat.

Warm the mixture over low heat, stirring often with a heat-safe spatula or wooden spoon. Scrape the bottom and sides of the pan as the chocolate softens and begins to melt into the condensed milk. Continue until the mixture is mostly melted and glossy.

When only small bits of unmelted chocolate remain, remove the saucepan from the heat. Stir until all the chocolate is completely melted and the mixture is smooth and shiny, with no lumps.

Let the mixture cool for 1–2 minutes off the heat so it is hot but not scorching. Stir in the Kahlua, vanilla extract, and fine sea salt. The mixture may thicken slightly when the cooler liqueur is added; keep stirring briskly until smooth.

If using chopped toasted nuts, mini chocolate chips, or chopped espresso beans, gently fold them into the fudge mixture until evenly distributed.

Pour or scrape the thick fudge mixture into the prepared pan. Use an offset spatula or the back of a spoon to spread it evenly into the corners and smooth the top.

If desired, sprinkle flaky sea salt, additional nuts, or mini chocolate chips over the top of the fudge and gently press them in so they adhere.

Cover the pan loosely with plastic wrap or foil and refrigerate for at least 2 hours, or until the fudge is firm enough to slice. For best texture and flavor, chill overnight.

Use the parchment sling to lift the chilled block of fudge out of the pan and transfer it to a cutting board. With a sharp knife, cut into 1- to 1.5-inch squares, wiping the blade with a warm, damp cloth between cuts for clean edges. Serve chilled or at cool room temperature.