Preheat the oven to 350°F (175°C). In a medium bowl, stir together the flour, granulated sugar, and 3/4 cup of the chopped pecans.
Pour in the melted butter and mix until the crumbs are evenly damp and start to clump. Tip into the 9x13-inch dish and press into an even layer across the bottom.
Bake for 16 to 18 minutes, until the crust looks set and lightly golden at the edges. Set on a rack and let cool completely, about 30 minutes.
In a large bowl, beat the softened cream cheese with a hand mixer on low until completely smooth with no lumps. Add the powdered sugar and beat again until combined.
Gently fold in 1 cup of the thawed whipped topping with a spatula, using a light hand so the mixture stays airy.
Spread this layer over the fully cooled crust in an even sheet, right to the edges.
In a separate bowl, whisk the instant chocolate pudding mix with the cold milk and vanilla extract for about 2 minutes, until the pudding starts to thicken. Let stand for 3 to 5 minutes so it firms up to a spreadable set.
Spoon the pudding gently over the cream cheese layer and smooth it flat with an offset spatula, taking care not to drag the layer underneath.
Spread the remaining whipped topping (about 2 cups) over the pudding in a smooth layer.
Scatter the 1/4 cup reserved chopped pecans over the top. Shave the semisweet chocolate bar over the surface with a grater to finish.
Cover the dish and refrigerate for at least 4 hours, or overnight, before slicing into 12 squares.