Preheat oven to 400F degrees. Prepare a muffin pan with cupcake liners. Set aside.
In a bowl, microwave butter and peanut butter for 30 seconds and stir well. If not fully melted, microwave again until melted (you should not need another 30 seconds).
Add sugar, milk and vanilla extract, stir to incorporate. Add eggs and mix.
With a spatula, mix in dry ingredients until there are no dry streaks.
Divide evenly into 12 muffin cavities (I use a 3-tablespoon scoop to start then evenly divide any remaining batter).
Make a small cavity to fit about 1 tablespoon (can do a little less) of jam in the middle of each muffin (if you want a very neat muffin make sure the jam is fully in the cavity, otherwise it will run down the sides).
Bake for about 13-14 minutes or until golden.
Remove from the oven and cool for several minutes on a wire rack before removing from pan.