Preheat your oven to 180°C (350°F) and line your muffin tin with paper liners or lightly grease it.
In a large mixing bowl, whisk together the flour, sugar, and baking powder until well combined. This creates a light, aerated dry mixture.
Add the peanut butter to the bowl and stir it into the dry mixture. It should start to break apart and disperse evenly.
In a separate bowl, whisk the eggs until they are smooth and slightly frothy. This helps incorporate air for a fluffy crumb.
Pour the eggs into the dry ingredients, along with the milk and melted butter. Gently fold everything together until just combined; the batter will be slightly lumpy but should come together smoothly.
Spoon the batter evenly into the prepared muffin cups, filling each about three-quarters full. The batter should mound slightly above the rim for a domed top.
Bake in the preheated oven for 20-22 minutes, until the muffins turn golden brown and a toothpick inserted in the center comes out clean.
Remove the muffins from the oven and let them cool in the pan for 5 minutes. Then, transfer them to a wire rack to cool completely, which helps set their tender crumb.
Once cooled, serve the muffins warm or at room temperature. For an extra treat, drizzle with honey or sprinkle with sea salt.