Preheat oven to 400°F. Line a 12-hole muffin pan with cupcake liners.
Whisk together the flour, baking powder, and salt.
In a large bowl, whisk together the eggs and sugar. Add the buttermilk, oil, vanilla, and peanut butter, and whisk until combined
Add the dry ingredients and fold in until just combined. Do not overmix.
Spoon the batter only halfway in the muffin pan. Add about 1 teaspoon of jam to the center of the batter, then top with the remaining batter, filling to the top.
Next, add a little more jam on the tops of the muffins, and create swirls by running a knife or toothpick through it. (Note: you can microwave the jam until slightly warm to make it easier to swirl into the batter.)
Bake for 15 - 20 minutes, or until a toothpick inserted into the center comes out clean.
Transfer the muffins to a wire rack to cool completely.