Lightly grease a 9×13″ baking dish with cooking spray to prevent sticking, and set aside. Or line with parchment paper.
In a medium bowl, whisk together the dry ingredients: flour, pumpkin pie spice, and salt. Set aside.
In a separate large mixing bowl, mix the melted butter with the light brown sugar and granulated sugar until well combined. Add the egg and vanilla extract to the sugar-butter mixture, and beat until smooth and evenly blended. Stir in the milk and canned pumpkin until fully incorporated.
In a separate large bowl, beat together the wet ingredients (butter, granulated sugar, and brown sugar). Mix until completely smooth and no lumps remain.Then, add the egg, vanilla extract, pumpkin puree and milk. Mix until the mixture is smooth and evenly blended.
Gradually add the dry ingredients to the wet ingredients, mixing just until combined. Be careful not to overmix, as this can lead to dense blondies.
Optional: Fold in the butterscotch chips, chocolate chips, and chopped pecans, if using.
Pour the batter into the prepared baking pan, and use a spatula to spread it out evenly. If desired, sprinkle with chopped pecans. Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted into the center comes out clean. Allow the blondies to cool completely for at least 10 minutes or to room temperature in the pan on a wire rack before slicing into squares.