Make the Custard: In a medium bowl, whisk the egg yolks and caster sugar together until pale and slightly thickened. Whisk in the cornflour until no lumps remain. Meanwhile, heat the milk and vanilla bean paste in a small saucepan over medium heat until it just begins to simmer.

Temper and Thicken: Slowly pour about half of the hot milk into the egg yolk mixture, whisking constantly to temper the eggs. Pour the egg mixture back into the saucepan with the remaining milk. Cook over a medium-low heat, whisking constantly, for 3-5 minutes until the custard thickens significantly and can coat the back of a spoon.

Chill the Custard: Remove the pan from the heat and whisk in the cold butter until fully incorporated. For an extra-smooth texture, I always pass the custard through a fine-mesh sieve into a clean bowl. Press a piece of cling film directly onto the surface of the custard to prevent a skin from forming. Refrigerate for at least 1.5 hours, or until completely cold and firm.

Prepare the Cookies: Preheat your oven to 180°C (160°C Fan) and line two baking trays with parchment paper. In a large bowl, using a stand mixer or electric hand mixer, cream the room-temperature butter and caster sugar together for 3-4 minutes until light and fluffy.

Mix the Cookie Dough: Beat in the egg and vanilla extract until well combined. In a separate bowl, whisk together the plain flour, baking powder, and salt. Add half of the dry ingredients to the butter mixture, mixing on low speed until just combined. Pour in the milk, mix briefly, then add the remaining dry ingredients. Mix only until no dry streaks remain—do not overmix.

Bake the Cookies: Drop rounded tablespoons of dough onto the prepared baking trays, leaving about 5cm of space between each one for spreading. I find that a small ice cream scoop works best for uniform size. Bake for 12-14 minutes, or until the edges are just beginning to turn golden. Let them cool on the tray for 5 minutes before transferring to a wire rack to cool completely.

Assemble the Cookies: Once the cookies are cool and the custard is firm, pair up cookies of a similar size. Pipe or spoon a generous amount of cold custard onto the flat side of one cookie and gently press another cookie on top to create a sandwich. Place the assembled cookies on a tray and pop them in the fridge while you make the glaze.

Make the Chocolate Glaze: Place the finely chopped dark chocolate in a heatproof bowl. Heat the double cream in a small saucepan until it's just simmering (don't let it boil). Pour the hot cream over the chocolate and let it sit for 2-3 minutes without stirring. Add the golden syrup if using, then gently stir until the ganache is smooth and glossy.

Glaze and Set: Let the ganache cool for about 10 minutes until it has thickened slightly. Take each chilled cookie sandwich and dip the top cookie into the ganache. Place it back on the wire rack (with parchment underneath to catch drips). Allow the glaze to set at room temperature for about 30 minutes, or speed it up in the fridge for 10 minutes before serving.