Line an 8x8-inch (20x20 cm) square baking pan with parchment paper, leaving overhang on two sides to create "handles" for lifting the fudge out later. Lightly spritz the parchment with nonstick spray if you want very clean edges.

In a medium heavy-bottomed saucepan, combine the semi-sweet chocolate chips, sweetened condensed milk, and butter. Set over low to medium-low heat and stir slowly and constantly with a heat-safe spatula or wooden spoon until the mixture is mostly melted.

Continue stirring gently until the mixture becomes thick, smooth, and glossy with no visible chocolate chunks. Scrape the sides and bottom of the pan often so nothing sticks or scorches. Remove from heat as soon as everything is fully melted.

Remove the saucepan from the heat. Stir in the fine sea salt and pure vanilla extract until fully combined. Taste a tiny bit carefully (it will be warm) to check the sweetness and chocolate flavor.

Pour in the amaretto liqueur and stir steadily and briskly until the mixture is thick, shiny, and homogeneous again. It may loosen slightly at first, then thicken as you stir. If using, gently fold in the chopped toasted almonds or pecans.

Scrape the fudge mixture into the prepared pan. Use a spatula to spread it evenly into the corners. The mixture will be thick but spreadable, like soft peanut butter. Tap the pan lightly on the counter to help the fudge settle and release air bubbles.

If desired, sprinkle the finely chopped almonds and a pinch of flaky sea salt evenly over the top of the fudge. Press very lightly so they adhere without sinking into the mixture.

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 cleanly. If your fridge is very full, chill 3–4 hours or overnight.

Once fully set, lift the fudge slab out of the pan using the parchment handles and place it on a cutting board. Use a long, sharp knife warmed briefly under hot water and wiped dry to cut the fudge into about 1-inch squares, wiping the knife between cuts for neat edges.

Serve the amaretto fudge squares slightly chilled or at cool room temperature for the creamiest texture. Store leftovers in an airtight container in the refrigerator for up to 2 weeks, with parchment or wax paper between layers.