Prepare the Pan: Line an 8x8 inch square pan with parchment paper, leaving overhang for easy removal.
Heat-Treat Flour: Microwave flour in 30-second intervals, stirring between each, until it reaches 165°F (about 1-2 minutes total). Alternatively, bake at 300°F for 5 minutes. Let cool completely.
Make Brownie Base (Optional): Whisk melted butter with both sugars until smooth. Add vanilla, then cocoa powder until no lumps remain. Stir in heat-treated flour and salt. Press firmly into prepared pan and refrigerate while making cookie dough.
Make Cookie Dough: In a large bowl, beat softened butter with both sugars until light and fluffy (2-3 minutes). Beat in milk and vanilla extract.
Add Dry Ingredients: Gradually mix in heat-treated flour and salt until soft dough forms. If too dry, add milk 1 tablespoon at a time.
Fold in Chocolate Chips: Gently fold in ¾ cup mini chocolate chips, reserving some for topping if desired.
Assemble: Spread cookie dough evenly over brownie base (if using) or directly into prepared pan. Press down firmly with the back of a spoon or measuring cup.
Chill: Refrigerate for 1.5-2 hours or freeze for 30 minutes until completely firm.
Add Chocolate Topping (Optional): Melt chocolate chips with cashew butter and coconut oil in microwave in 30-second intervals until smooth. Drizzle over chilled bars and sprinkle with sea salt.
Final Chill: Refrigerate 15-20 minutes to set chocolate topping.
Cut and Serve: Use parchment overhang to lift bars from pan. Cut into 16 squares with a sharp knife. Serve chilled.