Peanut Butter Bars

Nonstick cooking spray, for baking dish
1 1/2 cups confectioners' sugar
1 1/2 cups graham cracker crumbs
1 cup creamy peanut butter
Kosher salt
1 stick (8 tablespoons) unsalted butter, melted
8 ounces semisweet baking chocolate, chopped into small pieces
1 cup heavy cream
1 teaspoon flaky sea salt, optional