Preheat oven to 350 degrees F (175 degrees C). Grease and flour 24 mini-muffin cups.

Whisk flour, sugar, baking powder, baking soda, and salt together in a bowl.

Stir buttermilk, egg, syrup, and butter together in a separate large bowl until just combined; add flour mixture and stir with a spoon until just combined and no lumps remain. Fold peanut butter pieces into the mixture; spoon into the prepared muffin cups.

Bake in the preheated oven until a toothpick inserted into the center comes out clean, 9 to 12 minutes. Cool in the pans for 3 minutes before serving.