Preheat oven to 350F/180C. Grease or line a muffin tin.
Beat the peanut butter, butter and sugar together with an electric whisk or stand mixer for a few minutes until light and fluffy. Beat in the eggs, one at a time, beating well after each one, then beat in the vanilla and the milk. Sift in the flour, baking powder and salt and gently combine.
Spoon the mixture into the muffin cases, filling each ¾ full.
Bake for 20 minutes or until an inserted skewer comes out clean.
Allow to cool for a few minutes in the tin and then transfer to a wire rack to cool completely before frosting with the strawberry jam icing.
For the strawberry jam icing
Press the strawberry jam through a fine mesh sieve to remove any large pieces, collecting the thinner jam in a small bowl. You should have 1 tablespoon of jam in the bowl to use.
Beat the butter, sugar and sieved strawberry jam with an electric whisk or stand mixer until smooth. Keep in the fridge until ready to frost the cupcakes.