Preheat the oven to 325°F. Have ready an ungreased 13 × 9 × 2-inch baking pan.
Whisk together in a medium bowl:
Add:
Using a pastry blender or your fingertips, cut or rub in the butter until it is the size of small peas. Press the dough into the bottom and ¾ inch up the sides of the baking pan. Bake until golden brown, 25 to 30 minutes. Transfer the pan to a rack and reduce the oven temperature to 300°F.
Whisk together in a large bowl until well combined:
Stir in:
Sift over the top and stir in until well blended and smooth:
Pour the batter over the baked crust. Return to the oven and bake until the topping is set, about 35 minutes. Let cool in the pan on a rack before cutting into bars.