Line an 8x8 baking pan with foil, spray with nonstick baking spray, and set aside.
Add white chocolate chips, sweetened condensed milk, and butter to a saucepan. Heat ingredients over medium-low heat, stirring constantly, until melted and smooth. Do not turn up the heat as it could seize the chocolate or burn the pan.
Remove melted white chocolate from stove and pour evenly into the prepared pan. Spread evenly with a spatula. Refrigerate until set, which is about 4 hours.
When ready to serve, lift by the edges of the foil and pull the fudge from the pan. Peel foil away from fudge and discard.
Using a sharp knife, cut cold fudge into small cubes. If pulling from freezer, its best to let come to room temperature for best taste and texture. Serve or store.
Notes
Note on setting:To set quicker, place fudge in freezer. As a side note, be sure pan is flat. It is helpful to sit it on a baking sheet if placing inside a freezer.Note on storage: While fudge can be stored in an airtight container at room temperature, many kitchens and warmer climates can soften, so I store mine in the fridge.