Prep the pan. Line an 8-inch square baking pan with parchment paper, leaving overhang on two sides for easy lifting. Lightly mist with nonstick spray if you like.

Combine the base. In a medium, heavy-bottomed saucepan, add the chocolate chips, sweetened condensed milk, and butter.

Gently melt. Set the pan over low heat. Stir slowly and constantly with a heatproof spatula until the chocolate is almost fully melted and the mixture looks smooth and glossy. Remove from heat while a few small bits remain; they’ll melt off-heat.

Flavor it. If using add-ins like vanilla (1 teaspoon) or a pinch of salt, stir them in now. Fold in any mix-ins (nuts, marshmallows, etc.) quickly so they don’t melt away.

Spread and smooth. Scrape the mixture into the lined pan. Use the spatula to level the top. For a clean finish, tap the pan firmly on the counter a couple of times to release air bubbles.

Chill to set. Refrigerate for 2–3 hours, or until firm enough to slice cleanly. For best texture, don’t rush this step.

Slice and serve. Lift the fudge out using the parchment. Warm a sharp knife under hot water, wipe dry, then cut into 1-inch squares. Warming the knife gives super clean edges.