Preheat oven to 375 degrees. Cream peanut butter, butter, and both types of sugar together. Add egg, vanilla, flour, baking soda and salt. Mix all of these ingredients well. Roll into small balls that fill the openings in a small muffin pan about 2/3 full. Place into greased small muffin pan. Bake for 8 to 10 minutes at 375 degrees. Have candy unwrapped and place one Reese's cup into the middle of each cookie as soon as you remove them from the oven