Preheat oven to 350 degrees F. Line an 8x8 baking dish with parchment paper. Leave an over hang on two sides of baking dish for easy removal. Lightly spray parchment paper with non-stick cooking spray. Set aside.
In a medium bowl, add the flour, baking soda and salt. Whisk together and set aside for later.
In a stand mixer with the paddle attachment, cream together the butter, brown sugar and granulated sugar, until light and fluffy, about 2 minutes. Ladle sides of mixing bowl.
Add egg and vanilla extract, making sure both are incorporated well.
With the stand mixer on low, add the flour mixture slowly. Mix until just incorporated.
Fold in 3/4 cups Thin Mints and 1/4 cup white chocolate chunks.
Spread batter into prepared baking dish. Wet your fingertips just a little with water and spread batter evenly in baking dish.
Sprinkle remaining Thin Mints and white chocolate over top of the dough.
Bake for 25-30 minutes or until tooth pick inserted comes out with little crumbs. Place on top of the stove until cooled. Remove from pan and peel off parchment paper. Cut into squares and serve.