Preheat oven to 350° F
Sift or whisk together flour, baking powder and salt.
Place softened butter and granulated sugar (measure #1) in bowl of a stand mixer. Using the paddle attachment begin mixing on medium speed. Continue to mix until the butter and sugar mixture is light and fluffy (approximately 2 - 3 minutes). Note: a hand mixer may also be used
Add the flour, baking powder and salt mixture. Mix for 20 - 30 seconds.
Remove the bowl from the mixer and, using a rubber spatula, mix the dough briefly. Note: the dough will be crumbly.
Pour the dough into the bottom of the prepared pan. Using your hands, lightly press the dough down so it evenly covers the bottom of the pan.
Bake for 12 minutes.
Place on a cooling rack, cool for 10 minutes then proceed with making the topping.