Begin by bringing the butter to room temp by leaving it out on the counter for 20 minutes or so.
Mix the softened butter with the sugar until light and fluffy.
Add the flour, baking powder, and salt, then mix well.
Place an ungreased sheet in the pan
Pour the dough into a baking pan, wrap in plastic and refrigerate for 30 minutes.
Press with fingers into 1/4 inch thickness. The key to successful shortbread cookie dough is to handle it as little as possible. Don’t pound or knead it heavily or this can affect the levity of the shortbread.
Using a sharp knife or pizza cutter, cut into rectangles. You can also use a cookie cutter, if that's easier for you.
Poke a fork carefully into the little rectangles, creating holes. This helps with even cooking. Place in the refrigerator for thirty minutes.
Bake for 20 minutes at 350 °F (175 °C).
Remove to the cooling rack, and serve.
