Preheat oven to 375 degrees.
In a bowl, cream together the butter 1/2 cup sugar, and brown sugar until light and fluffy. Add in the peanut butter, egg, and vanilla. Mix until creamy.
Add in the dry ingredients: flour, baking powder, and baking soda. Stir until just combined.
Roll the dough into 1 inch balls, and roll in the remaining sugar. Place on a parchment lined baking sheet.
Using a fork, make a cross hatch pattern by pressing down in both directions.
Bake for 8-12 minutes, or until lightly golden brown on the edges. Let sit on the pan for 5 minutes, then move to wire rack to cool.
In a bowl, combine the chocolate and oil, and microwave 15 seconds at a time, stirring, until melted.
Dip half of each cooled cookie into the chocolate, and top with desired sprinkles or nuts.
