Lightly grease an 8- or 9-inch square baking pan with butter. Line it with parchment paper, letting the paper hang over two opposite sides to create handles for easy removal.

In a medium saucepan over low heat, combine the semi-sweet chocolate chips, 1 full 14 oz can of sweetened condensed milk (reserving 3/4 cup for the cappuccino layer if you prefer to measure it out first), and 2 tablespoons unsalted butter. Stir continuously with a silicone spatula until the chocolate is fully melted and the mixture is smooth and glossy.

Remove the saucepan from the heat and stir in 1 teaspoon vanilla extract and 1/4 teaspoon fine sea salt. The mixture should be thick, shiny, and pourable, similar to warm brownie batter.

Pour the chocolate fudge mixture into the prepared pan and spread it into an even layer, pushing it gently into the corners. Set aside at room temperature while you prepare the cappuccino layer. If your kitchen is very warm, chill the pan in the refrigerator for about 10 minutes to let the layer begin to set.

In a small bowl, stir together the instant espresso powder (or instant coffee granules) and 2 tablespoons very hot water until completely dissolved. Set aside.

In a clean saucepan over low heat, combine the white chocolate chips, 3/4 cup sweetened condensed milk (measured from the can), and 2 tablespoons unsalted butter. Stir constantly until the mixture is melted and smooth. White chocolate scorches easily, so keep the heat low and do not walk away.

Remove the saucepan from the heat and stir in the dissolved espresso mixture, 1 teaspoon vanilla extract, and 1/4 teaspoon fine sea salt. If the mixture seems extremely thick, you can add 1–2 teaspoons of hot water to loosen slightly; it will firm as it chills.

Pour the cappuccino fudge mixture over the chocolate layer in the pan. Working fairly quickly, spread it into an even layer. For a swirled effect, gently drag a butter knife through both layers in a few places to marble them lightly, taking care not to overmix.

Cover the pan loosely with foil or plastic wrap and refrigerate for at least 3 hours, or until the fudge is completely firm. For the cleanest cuts and most distinct layers, chill overnight.

Use the parchment paper handles to lift the chilled fudge slab out of the pan and set it on a cutting board. With a long, sharp knife, cut into 1-inch squares for bite-size pieces, or larger pieces if desired. Wipe the knife with a warm, damp cloth between cuts for neat edges.

If desired, lightly dust the top of the fudge with cocoa powder, sprinkle a pinch of flaky sea salt, and/or press crushed chocolate-covered espresso beans into the top. Serve slightly chilled or at cool room temperature.