Prepare Your Cookie Dough: In a large mixing bowl, beat the softened butter with both the caster sugar and light brown sugar using an electric mixer until the mixture becomes light and fluffy, taking on a pale, creamy hue (about 2-3 minutes). This aeration is key for a tender cookie.

Add Wet Ingredients: Beat in the egg and vanilla extract until fully incorporated. The mixture will look smooth and slightly glossy. Scrape down the sides of the bowl to ensure everything is evenly combined.

Combine Dry Ingredients: In a separate bowl, whisk together the plain flour, bicarbonate of soda, and sea salt. Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix; stop as soon as no streaks of flour remain, and the dough forms a soft, pliable ball.

Fold in Chocolate: Gently fold in the chocolate chips and chunks until they are evenly distributed throughout the dough. The dough will now be studded with delightful pockets of chocolate. Cover the bowl and chill the cookie dough in the refrigerator for at least 30 minutes. This helps prevent spreading and deepens the flavours.

Preheat Oven and Prepare Croissants: Preheat your oven to 180°C (160°C fan/Gas Mark 4). Line a baking tray with parchment paper. Take your chilled ready-to-bake croissants and carefully slice them horizontally, almost all the way through, creating a pocket without fully separating the halves. You want them to look like open clam shells, ready for filling.

Fill and Bake: Take portions of your chilled cookie dough (about 60-70g per crookie) and gently press them into the opened croissant pockets. Don't be shy; really fill them up! You can also press some cookie dough onto the top of the croissant. Place the filled croissants onto the prepared baking tray, leaving ample space between them.

Bake Until Golden: Bake for 15-20 minutes, or until the croissants are beautifully golden brown and incredibly flaky, and the cookie dough inside is set at the edges but still wonderfully gooey in the centre. You'll notice the aroma of warm butter and melting chocolate filling your kitchen. If desired, sprinkle with flaky sea salt immediately after removing from the oven.

Cool and Serve: Allow the Crookies to cool on the baking tray for 5-10 minutes before transferring them to a wire rack. This brief cooling period helps the cookie set slightly, making them easier to handle while still warm and melty. Serve warm for the ultimate experience.