Preheat your oven to 180°C (350°F). Line a muffin tin with paper liners or lightly grease it.
In a large mixing bowl, whisk together the flour, baking powder, salt, and sugar until evenly combined. The mixture should look light and slightly fluffy.
In a separate bowl, whisk the egg until it's pale and slightly frothy. Then, add the milk and peanut butter, stirring until the mixture is smooth, creamy, and well combined with no lumps.
Pour the wet ingredients into the bowl with the dry ingredients. Use a spatula or spoon to fold them together gently, just until the batter is combined and no streaks of flour remain. Don't overmix; a few lumps are fine.
Spoon the batter into the prepared muffin cups, filling each about three-quarters full. The batter should be thick but scoopable.
Bake in the preheated oven for about 20-22 minutes. The muffins are ready when they turn golden on top and a toothpick inserted into the center comes out clean with no batter clinging.
Remove the muffins from the oven and let them cool in the tin for about 5 minutes. Then, transfer them to a wire rack to cool completely or serve warm for the best experience.
Enjoy your soft, nutty peanut butter muffins as a quick breakfast or a satisfying snack. They’re best enjoyed fresh, but can be stored in an airtight container for up to a week or frozen for longer storage.