Preheat the oven to 400°F and grease 30 mini-muffin cups. Mix flours, salt, and baking powder in a bowl.
In another bowl, mash bananas. Add egg and sugar. Then mix in oil and milk until smooth. Slowly combine dry ingredients with wet ingredients, taking care not to overmix the batter.
Using a small ice-cream scoop or spoon, drop 1 tablespoon of batter into each mini-muffin cup and top with 1/2 teaspoon of peanut butter. Place 1 teaspoon of batter on top of the peanut butter to cover. Bake for 13 minutes, or until dry on top. (If you use regular muffin cups, bake for 20 minutes.) Spread jam or jelly on top just before serving, if desired. These muffins keep for a day covered at room temperature or up to 4 days refrigerated.