Preheat oven to 177°C (350°F) and line a 12-cup muffin pan with muffin liners, set aside.
To avoid any lumps, place flour, cocoa powder, baking soda and salt in a sifter or fine-mesh strainer and sift into a medium bowl, set aside.
Mash bananas in a large bowl. Whisk in melted butter, then sugar, egg and vanilla paste. Beat until well incorporated. Fold in dry ingredients with a spatula, stir just until combined.
Spoon about half of the mixture into the muffin pan, so the cups are about 1/3 filled. Make little dents into the centre of each one with the back of a spoon. Add a tablespoon of Nutella into each dent. Top with remaining muffin batter.
Lastly, decorate each muffin with pearl sugar. Bake muffins in the centre of the oven for 18-20 minutes or until the tops look puffed up and cracked and a tester inserted in different sides of each muffin comes out clean. (If you stick it directly into the centre it will get Nutella all over, so test muffins more closer to the edge.)
Let muffins cool in the pan for a few minutes, then take them out and let cool for a while longer.
Store muffins in an airtight container (or ziplock bag) at room temperature for up to 3-4 days.