Process flour, sugar, and cocoa powder in a food processor until combined. Add butter and process until mixture is crumbly. Press into the bottom of a 9x13-inch baking pan.
Bake in the preheated oven until lightly browned, about 15 minutes.
Combine condensed milk and 1 cup chocolate chips in a bowl. Microwave 1 minute. Stir; continue microwaving in 20-second intervals until chocolate is melted and mixture is smooth. Add vanilla extract and stir.
Pour chocolate and milk mixture over the baked crust. Top with remaining chocolate chips and walnuts.
Bake in the preheated oven until set, about 20 minutes. Let cool before cutting into 24 or 36 pieces.