Preheat the oven to 200C/400F/Gas 6. Line a 12-hole muffin pan with muffin cases, then set aside.
Whisk together the flour, baking powder, and salt. Set aside.
In a large bowl, whisk together the eggs and sugar until thick and light in colour. Add the buttermilk, oil, vanilla, and peanut butter, and whisk until combined.
Add the dry ingredients in thirds and fold in until just combined - do not overmix.
Spoon the batter into the muffin cases, filling only halfway. Add about 1 teaspoon of jam to the centres, then top with the remaining batter, filling to the tops.
Add a little more jam on the tops of the muffins, and create swirls by running a knife or toothpick through it (microwave the jam beforehand to make it easier to swirl into the batter).
Bake for 15 - 20 minutes, or until a toothpick inserted into the centre comes out clean.
Allow to cool for 5 - 10 minutes in the pan before transferring them to a wire rack to cool completely.
Video
Notes
*To make your own buttermilk, simply add 1 tablespoon of fresh lemon juice or white vinegar to 1 cup (240ml) of milk. Stir to combine, and allow to sit for 5 - 10 minutes until slightly thickened and curdled.Muffins stay fresh in an airtight container at room temperature for up to 5 days. They also freeze well for up to 2 months. Thaw overnight in the fridge.