Make the Cookies: In a large mixing bowl, cream the butter with an electric mixer until nice and creamy. Add the peanut butter and mix until it's just combined with the butter. Add the granulated and brown sugars, and mix until well creamed together with the butters. Add the egg and vanilla and mix until just combined. Add the baking powder, salt, and flour and mix, scraping the sides of the bowl as needed, just until the last streaks of flour disappear into the dough.
1/2 cup salted butter, softened at room temperature, 1/2 cup creamy peanut butter, 1/2 cup granulated sugar, 1/2 cup brown sugar, packed, 1 large egg, at room temperature , 1/2 tsp vanilla extract, 1/2 tsp baking powder, 1/4 tsp salt, 1 and 1/2 cups all-purpose flour, spooned & leveled or weighed out