Preheat the oven to 350°F and grease the inside of a 9 x 5 inch loaf pan.
To make the topping, stir the flour, peanuts, brown sugar, and salt together in a small bowl. Add the melted butter and stir until the dry ingredients are evenly saturated. Place the topping into the fridge to chill while you mix the batter.
In a medium bowl or container, whisk the milk, peanut butter, oil, vanilla, and apple cider vinegar together. Break up any large clumps of peanut butter and get the mixture relatively smooth — small flecks of peanut butter are fine, though.
In a large mixing bowl, whisk together the flour, sugar, brown sugar, baking powder, baking soda, and salt.
Pour the milk mixture into the bowl with the flour mixture, then stir just until the batter is combined. Don't overmix it — stop as soon as you no longer see flour streaks.
Transfer the batter to the prepared baking pan and smooth out the top with a spatula. Take the topping out of the fridge and sprinkle it over the batter, breaking up any large clumps with your fingers.
Bake the loaf for 50 to 55 minutes, until the topping is lightly browned and a toothpick inserted into the center comes out clean. The loaf will only rise slightly — this is totally fine.
Place the loaf pan on a cooling rack and let the bread cool completely.
Once the loaf is cool, loosen the sides with a knife, then remove it from the pan. Slice, serve, and enjoy.