Step 1: For the crust, combine crushed shortbread cookies, chopped pecans, and melted butter in a medium bowl. Press firmly into the bottom of a 9x13 inch baking dish. Refrigerate for at least 30 minutes to set.
Step 2: In a large bowl, beat the softened cream cheese, powdered sugar, and 1/4 cup milk with an electric mixer until smooth and creamy.
Step 3: Gently fold in 1 1/2 cups of the thawed whipped topping into the cream cheese mixture until fully combined. Spread this layer evenly over the chilled crust.
Step 4: In another large bowl, whisk together the instant butterscotch pudding mixes and 3 cups cold milk for 2 minutes, or until the pudding starts to thicken. Let it sit for 2-3 minutes to become firm.
Step 5: Carefully spread the butterscotch pudding layer over the cream cheese layer.
Step 6: Spread the remaining whipped topping over the butterscotch pudding layer. Cover the dish loosely with plastic wrap.
Step 7: Refrigerate the Butterscotch Lush for at least 3 hours (preferably 4-6 hours or overnight) to allow all the layers to set completely and the flavors to meld.
Step 8: Before serving, you can optionally drizzle with caramel sauce and garnish with extra pecans or toffee bits. Slice and serve cold.