In a double broiler, melt butter and chocolate over simmering water, stirring frequently, being careful not to burn.
Remove from heat and add sugar to bowl.
Stir sugar into chocolate until well blended. Let cool to just warm.
Add egg and vanilla to bowl.
Stir into chocolate mixture until smooth.
Add flour, baking soda and salt to chocolate mixture.
Stir together until just combined.
Spoon a tablespoon of batter into each greased muffin cup well.TIP: To insure that each brownie bite is the same size, I prefer to use a tablespoon cookie scoop when putting batter into each muffin cup.
Bake for 10 minutes until middle is almost set.
Immediately press a peanut butter cup into each brownie bite so that the top of the candy is flush with the top of the brownie.