Preheat your oven to 180°C (160°C fan) and line two large baking trays with parchment paper.

First, make the brownie cookies. Place the cubed butter and chopped dark chocolate in a heatproof bowl set over a small saucepan of simmering water (a bain-marie). Stir occasionally until completely melted and smooth. Set aside to cool for 5-10 minutes.

In a separate large bowl, using an electric hand mixer or a stand mixer with the whisk attachment, beat the eggs and light brown sugar together for 3-4 minutes until pale, thick, and voluminous. Whisk in the vanilla extract.

Gently pour the cooled chocolate mixture into the egg mixture and fold it through with a spatula until just combined. Be careful not to knock out too much air.

Sift the plain flour, cocoa powder, bicarbonate of soda, and a pinch of salt over the wet ingredients. Fold everything together until no floury streaks remain. The batter will be thick and glossy.

Drop heaping tablespoons of the batter onto your prepared baking trays, leaving at least 5cm of space between each one to allow for spreading. I find that using a medium ice cream scoop gives you perfectly uniform cookies, which is ideal for sandwiches. You should get about 24 cookies.

Bake for 10-12 minutes. The edges should be set and the tops will be cracked and shiny, but the centre should still be slightly soft. They will firm up as they cool. Leave them on the tray for 5 minutes before transferring them to a wire rack to cool completely.

While the cookies are cooling, make the buttercream. First, heat-treat the flour to kill any bacteria. Spread the flour on a baking tray and bake at 180°C (160°C fan) for 5 minutes, or microwave on high for 60-90 seconds until it reaches 74°C. Let it cool completely.

In a large bowl, beat the softened butter with an electric mixer until light and creamy. Add the light brown sugar and icing sugar and beat again for 3-5 minutes until very pale and fluffy.

Add the cooled flour, vanilla, milk, and salt, and beat on a low speed to combine, then increase the speed and beat for another 2 minutes until smooth. Fold in the miniature chocolate chips.

To assemble, pair up the cooled brownie cookies by size. Pipe or spoon a generous amount of the cookie dough buttercream onto the flat side of one cookie, then gently press another cookie on top to create your sandwich. Repeat with the remaining cookies.