Preheat the oven to 350°F (175°C). Grease a 9x5-inch loaf pan and line with parchment if desired.
In a large bowl, beat together the peanut butter, softened butter, and granulated sugar until creamy and smooth.
Add the eggs one at a time, mixing well after each, then stir in the vanilla extract.
In a separate bowl, whisk the all-purpose flour, baking powder, baking soda, and salt until combined.
Gradually add the dry ingredients to the wet mixture, alternating with the milk, beginning and ending with the dry ingredients; mix until just combined to form a thick batter.
Spoon about two-thirds of the batter into the prepared loaf pan and spread into an even layer.
Warm the Nutella slightly if stiff, then dollop it over the remaining batter and gently swirl a few spoonfuls of Nutella into the batter in the pan using a skewer or knife to create ribbons. Add remaining batter on top and swirl again to create a marbled effect.
Bake in the preheated oven for about 20 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs but no raw batter.
Allow the loaf to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely before slicing.