Grease an 8×8-inch baking pan with nonstick spray and line the bottom with parchment paper, extending the parchment up the sides for easy removal. Set aside.

Place the flour in a microwave-safe bowl and microwave for 1 minute, stirring halfway through to ensure even heating. Allow the flour to cool completely before using.

Place the mini chocolate chips in the freezer and keep them chilled until ready to incorporate. This prevents melting when added to the warm fudge mixture.

In a large mixing bowl, beat the softened butter, brown sugar, granulated sugar, milk, and vanilla extract with an electric mixer until smooth. Add the cooled flour and salt, then continue mixing until the dough is fully combined and uniform.

In a separate microwave-safe bowl, combine the white chocolate chips and sweetened condensed milk. Microwave in 30-second intervals, stirring between each interval, until the mixture is fully melted and smooth.

Pour the melted white chocolate mixture into the cookie dough mixture and stir until fully incorporated. Gently fold in the frozen mini chocolate chips to prevent melting.

Pour the mixture into the prepared pan, spreading it evenly with a rubber spatula. Refrigerate for at least 2 hours, or until fully set.

Once firm, lift the fudge from the pan using the parchment overhang and cut into squares. Store in the refrigerator until ready to serve.