Line an 8×8-inch (20×20 cm) square baking pan with parchment paper, leaving an overhang on two sides to create handles. Lightly grease the parchment with butter or nonstick spray to make removing and slicing the fudge easier.
In a medium, heavy-bottomed saucepan, add the semi-sweet chocolate chips, sweetened condensed milk, and butter pieces. Set the pan over low heat.
Stir constantly with a silicone spatula or wooden spoon as the mixture warms. Keep the heat low to prevent the chocolate from scorching or seizing. Continue stirring for about 5–7 minutes, until the mixture is fully melted, thick, shiny, and completely smooth with no streaks of unmelted chocolate.
Remove the saucepan from the heat and let it sit for about 1 minute. Pour in the Baileys Irish Cream, vanilla extract, and fine sea salt. Stir well until everything is fully combined and the fudge mixture looks glossy again. It may loosen slightly when you add the liqueur; keep stirring until it thickens back up and looks uniform.
Quickly scrape the warm fudge mixture into the prepared baking pan. Use the spatula to spread it evenly into the corners and smooth the top. If it seems very thick, gently tap the pan on the counter a few times to help the fudge settle and release air bubbles.
While the fudge is still soft, sprinkle mini chocolate chips, chopped toasted nuts, flaky sea salt, and/or festive sprinkles evenly over the surface. Press the toppings very lightly so they adhere without sinking too deeply into the fudge.
Transfer the pan to the refrigerator and chill for at least 2–3 hours, or until the fudge is firm enough to slice. For the cleanest slices and best flavor, chill for 4 hours or overnight to allow the fudge to fully set and the flavors to meld.
Once firm, use the parchment handles to lift the fudge slab out of the pan and onto a cutting board. With a sharp knife, cut into about 1-inch squares (or smaller, since the fudge is quite rich). Wipe the knife with a warm, damp cloth between cuts for neat, clean edges.
Serve the Baileys Irish Cream fudge as part of a dessert tray, gift box, or as an indulgent treat. Store leftovers in an airtight container at room temperature for up to 2 days (if your kitchen is cool) or in the refrigerator for up to 1 week. For longer storage, freeze for up to 2 months, separating layers with parchment.