Preheat oven to 375°F (190°C) and line a muffin tin with paper liners.
Whisk flour, sugar, baking powder, and salt in a bowl.
In another bowl, whisk peanut butter, melted butter, eggs, milk, and vanilla until smooth.
Combine wet and dry ingredients and mix just until incorporated.
Fill muffin cups halfway with batter.
Add 1 teaspoon jam to center of each muffin.
Cover jam with remaining batter until cups are 3/4 full.
Bake 16–20 minutes until tops turn golden and a toothpick comes out clean.
Cool slightly before serving.