Prepare the Filling: Line a small baking tray or plate with parchment paper. Dollop 12 teaspoon-sized mounds of Nutella (or your chosen filling) onto the paper. Place the tray in the freezer for at least 1 hour, or until the mounds are solid. This makes them much easier to wrap the dough around.

Brown the Butter: In a small, light-coloured saucepan, melt the unsalted butter over a medium heat. Continue to cook, swirling the pan occasionally, as the butter foams and then subsides. You'll see little brown bits forming at the bottom and smell a wonderful, nutty aroma. This takes about 5-7 minutes. Immediately pour the browned butter, including all the brown bits, into a large mixing bowl to stop the cooking process. Let it cool for 10 minutes.

Mix the Dough: To the bowl with the cooled browned butter, add the dark brown sugar and granulated sugar. Whisk vigorously for a minute until well combined. Add the egg, egg yolk, and vanilla extract, and whisk again until the mixture is glossy and smooth.

Combine Dry Ingredients: In a separate bowl, whisk together the plain flour, bicarbonate of soda, baking powder, and salt.

Finish the Dough: Add the dry ingredients to the wet ingredients and fold them together with a spatula until just combined. Be careful not to overmix. Fold in the dark chocolate chunks.

Chill the Dough: Cover the bowl with cling film and place it in the refrigerator to chill for at least 30 minutes. The dough will be firm and less sticky, making it easier to handle.

Stuff the Cookies: Preheat your oven to 180°C (160°C fan) and line two large baking trays with parchment paper. Using a large cookie scoop or your hands, portion the dough into 12 equal balls (each should be about 60-65g). Take one ball of dough, flatten it into a disc in your palm, place one frozen Nutella mound in the centre, and carefully wrap the dough around it, sealing the edges completely. Roll it back into a smooth ball. I find that a tight seal is key to preventing any filling from escaping during baking. Repeat with the remaining dough and filling.

Bake to Perfection: Place 6 cookie balls on each prepared tray, leaving plenty of space between them as they will spread. Bake for 12-14 minutes, or until the edges are golden brown and set, but the centres still look slightly soft.

Cool and Enjoy: Let the cookies cool on the baking trays for 10 minutes before transferring them to a wire rack to cool completely. This allows them to set up properly. They are fantastic when still slightly warm!