Line an 8×8-inch baking dish with parchment paper and lightly spray with nonstick spray.
Heat-treat the flour by microwaving it in a microwave-safe bowl for 1 minute. Let it cool completely, then sift to remove clumps.
Place 1 cup regular M&M’s and ½ cup mini M&M’s into zip-top bags and freeze while preparing the mixture.
In a mixing bowl, beat butter, granulated sugar, brown sugar, half and half, vanilla, and salt for 1½–2 minutes until creamy.
Reduce speed and mix in the cooled flour until just combined.
In a separate bowl, microwave white chocolate chips and sweetened condensed milk in 30-second intervals, stirring until smooth.
Stir the frozen M&M’s into the cookie dough mixture.
Add the melted white chocolate mixture and stir until fully combined.
Spread the mixture evenly into the prepared baking dish.
Sprinkle remaining M&M’s and mini M&M’s on top and gently press them in.
Cover and refrigerate for at least 2 hours (or overnight) until set.
Slice into 25 squares and serve.