Preheat your oven to 180°C (350°F) and line a muffin tin with paper liners or lightly grease it.
In a large mixing bowl, whisk together the flour, sugar, baking powder, and salt until evenly combined; this creates a fluffy, light base for the muffins.
In a separate bowl, beat the egg until it’s slightly frothy, then stir in the peanut butter, milk, and melted butter until the mixture is smooth and slightly creamy.
Pour the wet ingredients into the bowl with the dry ingredients, then gently fold them together with a spatula or whisk until just combined—be careful not to overmix, so the muffins stay tender.
Spoon the batter evenly into the prepared muffin cups, filling each about three-quarters full for a nice domed top.
Bake for 20-22 minutes, or until the muffins turn golden on top and a toothpick inserted into the center comes out clean with no wet batter.
Remove the muffins from the oven and let them cool in the tin for about 5 minutes—this helps them set and makes them easier to handle.
Transfer the muffins to a wire rack to cool completely, which allows excess steam to escape and keeps the tops from becoming soggy.
Once cooled, enjoy these moist, fragrant muffins warm or at room temperature—perhaps with a spread of extra peanut butter or a drizzle of honey.