Combine the graham cracker crumbs and light brown sugar in a medium bowl and stir to mix. Pour in the melted butter and stir until the crumbs look like wet sand and hold together when pressed.
Tip the mixture into your 9x13 dish and press it firmly and evenly across the bottom, using the flat base of a glass or measuring cup to compact it. Place the dish in the freezer for 15 to 20 minutes to set the crust while you make the next layer.
In a large bowl, beat the softened cream cheese on medium speed until completely smooth with no lumps. Add the confectioners sugar and vanilla extract, then beat again until fully combined and fluffy.
Add 1 cup of thawed whipped topping and beat on low speed just until the mixture is smooth and uniform, being careful not to overmix. Remove the crust from the freezer and spread the cream cheese layer gently over it in an even sheet, taking care not to pull up crumbs.
In a separate bowl, whisk both boxes of instant butterscotch pudding mix with 2.5 cups of cold milk. Whisk continuously for about 2 minutes until the pudding starts to thicken noticeably.
Check the texture. It should be thick enough to hold its shape on the whisk but still spreadable, and if it feels stiff you can whisk in the extra 1/2 cup of milk a splash at a time. Let the pudding sit for 3 to 4 minutes to finish setting, then spread it carefully over the cream cheese layer in an even layer.
Spoon the 2 cups of whipped topping over the pudding layer in dollops, then gently spread them together so you never drag the pudding up into the topping. Smooth the top flat with an offset spatula or the back of a spoon.
Sprinkle the chopped pecans evenly over the surface, and drizzle with butterscotch sauce or scatter butterscotch chips if using. Cover and refrigerate for at least 4 hours, or overnight, before slicing into squares and serving chilled.