Preheat your oven to 180°C (350°F). Line a muffin tin with paper liners or grease it well to prevent sticking.
In a large mixing bowl, whisk together the flour, sugar, and baking powder until evenly combined. This creates a light, airy base for the muffins.
In a separate bowl, whisk the peanut butter, milk, egg, and melted butter until the mixture is smooth and slightly frothy, with a fragrant aroma of peanut butter.
Pour the wet mixture into the bowl with the dry ingredients. Gently fold everything together with a spatula or whisk until just combined—lumps are okay, avoid overmixing to keep the muffins tender.
Spoon the batter evenly into the prepared muffin tin, filling each cup about two-thirds full. The batter should be thick but scoopable, with a slightly sticky texture.
Bake in the preheated oven for 20-22 minutes, or until the muffins are golden on top and a toothpick inserted into the center comes out clean. The tops will crack lightly and smell warm and nutty.
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 enjoy warm for a gooey, soft texture.
Serve the muffins warm or at room temperature, with a pat of butter or a drizzle of honey if desired. They’re perfect for a quick breakfast or an indulgent snack.