1. Heat oven to 375 degrees F. Place flour in a medium bowl. Break apart peanut brittle into a food processor. Pulse until the brittle is finely crushed (you should have about 3/4 cup). Add to bowl of flour, along with baking powder and salt. Set aside.
2. In a large mixing bowl, beat together butter and peanut butter until smooth. Add brown sugar and 1/2 cup of the granulated sugar. Beat until light in color, about 2 minutes. Add vanilla and eggs and beat until smooth, scraping down side of bowl. On low speed, beat in flour mixture.
3. Place remaining 1/2 cup sugar in a small bowl. Roll 1/4 cup dough into a ball; coat in sugar. Transfer dough balls to ungreased baking sheets and flatten with a 3-inch round ramekin. Use fork tines to make + marks on cookies.
4. Bake cookies in 375 degree F oven for 13 to 15 minutes or until edges are browned and cookies are set. Rotate pans halfway through baking. Transfer baked cookies to wire racks to cool. Repeat with remaining dough.
5. Store cookies at room temp in airtight container up to 2 weeks. Makes 22 cookies.