Preheat the oven to 350°F. Line a 9x9-inch baking pan with parchment paper. Set aside.
In a large bowl, combine the melted butter, brown sugar, and pumpkin.
Add the eggs, one at a time, stirring well after each addition.
Stir in the vanilla extract.
In a separate, medium-sized bowl, whisk together the flour, cornstarch, pumpkin pie spice, baking powder, and salt.
Add the dry mixture to the wet mixture and stir until completely combined.
Stir in the white chocolate chips.
Spread the batter evenly into the prepared pan. Bake for 40 minutes, or until the edges just begin to turn golden brown. (For an ooey-gooey cake, reduce the bake time by 5-8 minutes.)
Allow to cool before cutting and serving.