Line an 8x8-inch (20x20 cm) square baking pan with parchment paper, leaving overhang on two opposite sides to create handles. Lightly grease the parchment with a small amount of butter or nonstick spray so the fudge releases cleanly.

In a medium, heavy-bottomed saucepan, add the semi-sweet chocolate chips, sweetened condensed milk, and butter pieces. Place the pan over low heat. Keep the heat low to avoid scorching the chocolate and making the fudge grainy.

Stir constantly with a silicone spatula or wooden spoon as the chocolate melts, scraping the bottom and sides of the pan so nothing sticks. Continue cooking over low heat until the mixture is thick, glossy, and completely smooth, about 4–6 minutes.

Remove the pan from the heat. Stir in the vanilla extract, fine sea salt, and amaretto liqueur. The mixture may briefly look like it is separating; keep stirring until it comes back together into a shiny, thick fudge mixture, 20–30 seconds.

If using toasted sliced or slivered almonds or other mix-ins, gently fold them into the warm fudge mixture until evenly distributed. Avoid over-mixing.

Scrape the fudge mixture into the prepared parchment-lined pan. Use an offset spatula or the back of a spoon to spread it evenly into the corners and smooth the top. Work fairly quickly, as the fudge will begin to firm as it cools.

While the fudge is still warm and soft on top, sprinkle with mini chocolate chips, flaky sea salt, and/or a few extra almonds if desired. Press very gently so the toppings adhere.

Cover the pan lightly with plastic wrap or foil and refrigerate for at least 2 hours, or until the fudge is quite firm to the touch. For the cleanest cuts and best flavor, chill 3–4 hours or overnight.

Use the parchment handles to lift the fudge out of the pan and onto a cutting board. With a long, sharp knife, cut into small 1-inch squares, wiping the blade between cuts for neat edges. Serve slightly chilled or let stand at room temperature 10–20 minutes for a softer, creamier texture.