Line an 8×8-inch (20×20 cm) square baking pan with parchment paper, leaving overhang on two sides as handles. Lightly mist with nonstick spray if desired.
In a medium saucepan, combine 3 cups white chocolate chips, 1 can (14 oz) sweetened condensed milk, and 2 tablespoons unsalted butter.
3 cups white chocolate chips, 1 can sweetened condensed milk, 2 tablespoons unsalted butter
Warm the mixture over low heat, stirring constantly, until the white chocolate is mostly melted and the mixture is smooth. Keep the heat low to prevent the white chocolate from seizing.
3 cups white chocolate chips, 1 can sweetened condensed milk, 2 tablespoons unsalted butter
Remove from heat and stir in 2–3 teaspoons unsweetened cocoa powder, 1 teaspoon vanilla extract, and 1/4 teaspoon fine sea salt until no cocoa streaks remain. Add 1–1 1/2 teaspoons red gel food coloring a little at a time, stirring until you reach your desired red velvet color.
2-3 teaspoons unsweetened cocoa powder, 1 teaspoon pure vanilla extract, 1/4 teaspoon fine sea salt, 1-1 1/2 teaspoons red gel food coloring
Quickly pour the red velvet fudge into the prepared pan. Spread into an even layer and gently tap the pan on the counter to release any air bubbles.
In a medium mixing bowl, beat the softened cream cheese and 1 tablespoon unsalted butter with a hand mixer until very smooth and creamy, scraping down the sides of the bowl as needed.
4 oz cream cheese, 1 tablespoon unsalted butter
Gradually add 1 cup sifted powdered sugar, about 1/4 cup at a time, beating on low speed until fully incorporated and smooth.
1 cup powdered sugar
In a separate heat-safe bowl, gently melt 1 cup white chocolate chips in the microwave at 50% power in 20–30 second bursts, stirring between each, until smooth; or melt over a double boiler.
1 cup white chocolate chips
Beat the melted white chocolate, 1 teaspoon vanilla extract, and a pinch of salt into the cream cheese mixture until thick, smooth, and fudge-like. Work fairly quickly so it stays spreadable.
1 cup white chocolate chips, 1 teaspoon pure vanilla extract, salt
Drop spoonfuls of the cream cheese fudge mixture over the still-soft red velvet layer. Use a butter knife or skewer to gently swirl the white mixture into the red in a few figure-eight motions, being careful not to over-swirl.
If desired, sprinkle white nonpareils, heart sprinkles, or crushed cookies over the top and press lightly. Cover the pan and refrigerate for at least 2–3 hours, or until fully firm (overnight for cleanest slices).
white nonpareils or heart sprinkles, crushed red velvet or chocolate sandwich cookies
Lift the chilled fudge from the pan using the parchment handles and place on a cutting board. Use a sharp knife (warmed under hot water and wiped dry, if desired) to cut into about 36 small squares. Serve chilled or slightly cool.