Preheat your oven to 180°C (350°F). Lightly grease or line a muffin tin with paper liners. Gather all your ingredients for easy mixing.
In a large mixing bowl, whisk together the flour, brown sugar, baking powder, and salt until evenly combined. This creates a dry base for your batter.
In a separate bowl, beat the egg, then add the milk and vegetable oil. Whisk until smooth and slightly frothy, about 30 seconds.
Stir in the creamy peanut butter into the wet mixture until fully incorporated and smooth. The peanut butter should melt into the mixture, filling the air with a nutty aroma.
Pour the wet ingredients into the bowl with the dry ingredients. Gently fold everything together with a spatula or spoon until just combined. Do not overmix; the batter should be thick with some lumps.
Spoon the batter evenly into the prepared muffin cups, filling each about 3/4 full. Tap the muffin tin lightly on the counter to settle the batter and remove any air bubbles.
Bake the muffins in the preheated oven for 15–20 minutes. Check at the 15-minute mark; they should be golden on top and a toothpick inserted into the center should come out moist but clean.
Once baked, remove the muffins from the oven and let them cool in the tin for about 5 minutes. Then transfer them to a cooling rack to cool slightly, or serve warm if you prefer.
Enjoy these warm or at room temperature, perhaps with a pat of butter or a drizzle of honey for extra comfort. The muffins should be soft, moist, and fragrant with peanut aroma.