In a large mixer bowl, combine the flour, dry yeast, caster sugar, and egg. Gradually add the warmed milk and mix until a shaggy dough forms. Incorporate the softened butter and continue mixing until the dough is smooth and elastic. I like to knead dough for at least 8-10 minutes to develop the gluten for a light, airy texture. Cover the dough and let it proof in a warm place for about two hours, or until doubled in size. While the dough proofs, spoon generous dollops of Nutella onto a parchment-lined tray and freeze for at least 1 hour until solid.

Lightly flour a clean work surface. Gently turn out the proofed dough and knead it briefly to release any air bubbles. Roll the dough consistently to about 1 inch thick. Using a round cutter or glass, cut out circles of dough. Place a frozen Nutella dollop in the center of each dough circle, then carefully fold the dough over the Nutella, pinching the edges firmly to seal and form a ball. Make sure the Nutella is completely enclosed; it's a messy situation if it leaks during baking! Arrange the shaped dough balls on a baking tray, leaving enough space for them to rise.

Preheat your oven to 180°C (350°F). In a small bowl, whisk together the egg yolk and 1 tablespoon of milk to create an egg wash. Gently brush the tops of the dough balls with the egg wash for a beautiful golden-brown finish. Bake for approximately 20 minutes, or until the Nutella bombs are puffed and golden. I always bake until they're a deep golden for a nice crust that holds up to the soft interior.

Once baked, remove the Nutella bombs from the oven and let them cool slightly on the baking tray before transferring them to a wire rack. While still warm, dust generously with icing sugar, if desired. These are best enjoyed warm when the Nutella inside is gloriously melted and gooey!