1. In a bowl, whisk flour and salt. In a separate bowl, beat butter and sugar for 2 minutes, until fluffy. Add flour and beat to combine. Stir in white chocolate chips. Press firmly and evenly (the flat side of a measuring cup works well) into foil-lined pan. Refrigerate 15 minutes.Brownie layer
2. In a large bowl, whisk flour, cocoa powder, sugar and salt. Beat in melted butter, egg, milk and mint extract. Stir in semisweet chocolate chips. Dollop evenly over sugar cookie layer and spread with a spatula dipped in warm water (re-dip spatula as necessary to prevent sticking).
3. Bake at 350 degrees for 30 minutes. Cool. Remove from pan by lifting foil. Place on work surface. Melt white chocolate chips per package directions. Drizzle over bar cookies. Sprinkle with crushed mints. Allow chocolate to harden and slice into 36 pieces.