In a large bowl with electric mixer, or in a stand mixer, beat butter, peanut butter, granulated sugar, and 1/4 cup of brown sugar until light and fluffy.
2
Beat in egg and vanilla until thoroughly blended.
3
In another bowl, combine flour, baking soda, and salt and mix into the butter mixture until just combined.
4
Sprinkle the remaining 1/4 cup of brown sugar on top of the dough and lightly fold in with a spatula so the sugar granules are still visible.
5
Roll heaping teaspoons of dough into balls and place 2 in apart on an ungreased cookie sheet.
6
Using the bottom of a glass, press each ball into 1/4 in circle.
