Cream together 1 cup butter, 1 cup light brown sugar, and 1 cup granulated sugar.
Add 4 large eggs, one at a time, mixing well after each addition.
Stir in 2 teaspoons vanilla extract and 1 cup pumpkin puree until combined.
Add 2 cups white whole wheat flour, 3½ cups all-purpose flour, 1 teaspoon salt, 1 teaspoon baking soda and 3½ teaspoons pumpkin pie spice. Stir until just combined.
Spread into a 10x15-inch bar pan. The dough should be thick like cookie dough and thus a little hard to spread. Try to get it relatively uniform.
Bake for 15-20 minutes, until a toothpick inserted in the center comes out clean.
Allow to cool completely and then spread marshmallow buttercream or your favorite frosting over the top. MMM!
Notes
If you don't have white whole wheat flour, just use all-purpose flour instead.