Preheat an oven to 350 degrees F (175 degrees C). Line a 12 cup muffin pan with paper baking cups.
Whisk together the flour, baking powder, baking soda, and salt in a large bowl.
Beat the butter and sugar with an electric mixer in a large bowl until fluffy, about 1 to 2 minutes.
Add the egg and strawberry extract, continue beating. Beat in the milk and buttermilk, then beat in the jam. The batter should be pink.
Stir in 1/3 of the flour mixture, mixing until just incorporated. Stir in 1/2 of the remaining flour and then add the remaining flour; mixing just enough to evenly combine.
Pour the batter into prepared baking cups.
Bake in the preheated oven until a toothpick inserted into the cake comes clean, about 15 to 20 minutes. Cool before frosting.
Beat the peanut butter and butter until smooth in a large bowl.
Mix in 2 cups of the confectioners' sugar, 1/4 cup at a time; then stir in 1/4 cup milk. Gradually add the remaining 2 cups of sugar. If the frosting is too dry add a tablespoon of milk. Frost the tops of the cooled cupcakes.
