In a large bowl, stir peanut butter and sugar together until smooth. Beat in the eggs, then stir in the baking soda, salt, and vanilla. Place 1/2 cup sugar in a small bowl. Roll dough into 1 inch balls and then roll into sugar. Place the balls 2 inches apart onto a greased cookie sheet.
Bake for 8 to 10 minutes. Place Reese's peanut butter cup on top of the cookies after you remove them from the oven.