Preheat your oven to 375°F (190°C). Line a muffin tin with paper liners or lightly grease the cups.
In a large bowl, whisk together the flour, granulated sugar, brown sugar, baking powder, and salt.
In another bowl, whisk together the milk, peanut butter, vegetable oil, egg, and vanilla extract until smooth and well combined.
Pour the wet ingredients into the dry ingredients and stir until just combined. Be careful not to overmix; the batter should be slightly lumpy.
Divide the batter evenly among the prepared muffin cups, filling each about 3/4 full. If desired, sprinkle chocolate chips or chopped peanuts on top of each muffin.
Bake in the preheated oven for 15-20 minutes, or until a toothpick inserted into the centre of a muffin comes out clean.
Allow the muffins to cool in the tin for about 5 minutes before transferring them to a wire rack to cool completely.
Enjoy warm or at room temperature, perhaps with a drizzle of honey on top for extra sweetness.