Line an 8-inch (20 cm) square baking pan with parchment paper, leaving an overhang on two sides to create handles. Lightly spray the parchment with nonstick spray or butter it very lightly to make it easy to lift out the fudge and keep the edges neat.

In a medium, heavy-bottomed saucepan, combine the semi-sweet chocolate chips, sweetened condensed milk, and butter. Set over low heat and stir frequently with a heat-safe spatula or wooden spoon until the mixture begins to melt. Keep the heat gentle so the chocolate does not scorch or seize; if it seems to be melting too quickly, pull the pan partially off the burner and continue stirring.

Continue cooking over low heat, stirring constantly, until the mixture is fully melted, thick, and smooth with no visible chocolate chips remaining, about 5–7 minutes. Turn off the heat and keep stirring for a bit; the residual heat in the pan will finish melting any last bits.

Remove the saucepan from the burner and let it sit for about 1 minute to cool slightly. Stir in the Baileys Irish Cream, vanilla extract, and fine sea salt until fully combined. The mixture will loosen and look extra shiny at first; keep stirring until it thickens again to a texture similar to glossy chocolate frosting.

If using chopped toasted nuts, mini chocolate chips, or other mix-ins, fold them into the warm fudge mixture now, working fairly quickly so the fudge does not start to set in the pan.

Scrape the fudge mixture into the prepared pan. Use a spatula to spread it into an even layer, pushing it into the corners. Tap the pan firmly on the counter a few times to remove any trapped air bubbles and help the surface level out.

While the top of the fudge is still soft, sprinkle with flaky sea salt, extra mini chocolate chips, or chocolate shavings, if desired. Press gently with the back of a spoon so the toppings adhere.

Cover the pan loosely with plastic wrap or foil, avoiding direct contact with the surface if you want a perfectly smooth top. Refrigerate for at least 2 hours, or 3–4 hours (or overnight) for very clean slices, until the fudge is firm enough to cut.

Use the parchment handles to lift the chilled fudge out of the pan and onto a cutting board. With a sharp knife, cut into small squares about 1 to 1.5 inches each, wiping the blade with a warm, damp cloth between cuts for the neatest edges.

Arrange the Baileys Irish Cream fudge squares on a serving plate or pack them into candy boxes or tins. Serve slightly chilled or at cool room temperature. Store leftovers in an airtight container in the refrigerator for up to 2 weeks, layering parchment or wax paper between pieces to prevent sticking.