Prep. Line an 8×8-inch baking dish with parchment paper. Lightly butter and set aside.
Combine ingredients. In a large microwave-safe bowl, add 3 cups chocolate chips, 14 ounce can condensed milk, and 4 Tablespoons butter. Microwave 1 minute at 50% power. Stir and microwave in 30 second increments until the chocolate has melted, about 1 minute. Be careful not to over cook or it will become grainy.
Pour into pan. Quickly pour into the baking dish and press down with a spatula or piece of parchment paper. Refrigerate until set.
Store and slice. Store in the refrigerator until ready to serve. Slice into squares. I find it easiest to cut with a plastic knife.
Notes
Add-ins: Mix in 1/2-1 cup chopped walnuts or pecans immediately after the chocolate has melted.Toppings: Sprinkle crushed candy canes or flake sea salt on top immediately after you pour the chocolate into the pan.Storage: Store in a covered container for up to 1 week or in the freezer for up to 3 months.