Lightly grease an 8 or 9-inch square pan with butter then line it with parchment paper. The butter helps the parchment paper adhere to the pan. If you prefer, you can secure the parchment to the pan with binder clips or clothespins.

Combine the butterscotch chips, white chocolate chips, butter, and sweetened condensed milk in a large saucepan or skillet. Heat over medium-low, stirring constantly, until the chips are fully melted and the mixture is smooth. Remove the pan from the heat and immediately stir in the vanilla and chopped pecans. Pour into the prepared pan and smooth out the surface if needed. Sprinkle the flaky sea salt over the top.

Allow the fudge to sit at room temperature until firm in texture, about 2-3 hours. Once firm, slice into roughly 1 or 1.5-inch sized squares.