Mix flour, 1/2 cup pecans, and melted butter together in a bowl; pour mixture into a 9x13-inch baking dish and press to form a crust.
Bake in the preheated oven until crust is lightly browned, about 20 minutes. Remove from oven and cool completely.
Whisk cream cheese and confectioners' sugar together in a bowl until smooth; spread evenly over crust.
Beat vanilla pudding mix, chocolate pudding mix, and milk together in a bowl using an electric mixer until thickened; pour over cream cheese layer. Spoon whipped topping evenly over pudding layer.
Drizzle chocolate syrup over whipped topping layer making abstract designs; sprinkle with remaining 1/2 cup pecans. Refrigerate until chilled and set, about 2 hours.