Preheat oven to 350°F (177°C). Line a 9×13-inch baking pan with parchment paper with enough overhang on the sides to easily remove the bars from the pan. Set aside. Using a food processor, pulse the graham crackers (or crumbs) and pecans together into crumbs. A few larger pieces of nuts is OK. Add the sugar and melted butter and pulse until combined. Press tightly into the bottom and up the sides of the prepared baking pan. Pre-bake crust for 12 minutes. Leave oven on.  Using a handheld or stand mixer fitted with a whisk attachment (or simply using a whisk) beat/whisk all of the filling ingredients together in a large bowl until smooth. Pour into warm crust. Bake for 40–50 minutes or until the center is no longer jiggly. Remove from the oven, set on a wire rack, and allow to cool completely. Once cool, cover and place in the refrigerator for at least 1 hour and up to 3 days. Remove chilled bars from the pan using the overhang on the sides and cut into squares. Bars are best served chilled. Top with whipped cream and chopped pecans, if desired. Cover and store leftovers in the refrigerator for up to 1 week.