First, make the cookies. Tip the butter and both sugars into a stand mixer, or use an electric whisk to beat until pale and fluffy. Add the egg and vanilla paste, beating on a medium speed until combined. Tip in the flour and 1 tsp salt, then beat on a medium speed to combine. Be careful not to overwork the mix. Fold in the chocolate chips until evenly distributed.

Roll the dough into 12 cookies (weighing 60g each) and arrange over a baking tray lined with baking parchment. Cover with a tea towel and chill for 24 hrs, or freeze for 30 mins before baking.

Heat the oven to 180C/160C fan/ gas 4 and line two baking sheets with baking parchment. Put six cookies on each tray, well spaced apart. Bake for 10-12 mins until the edges are golden and the middles slightly paler. Cool completely. Will keep in an airtight container for two days, or frozen for three months.

To make the mousse, put 250g of the strawberries and the icing sugar in a food processor or blender, and blitz until completely smooth. Transfer to a small saucepan and warm over a low heat. Meanwhile, soak the gelatine leaves in a bowl of cold water for 2 mins. Remove the purée from the heat. While the purée is still warm, squeeze out the excess water from the gelatine and stir it into the purée until melted. Pour into another bowl and transfer to the fridge to cool quickly.

Whip the cream in a large bowl until it holds stiff peaks. Quarter the remaining strawberries and set aside. When the purée has cooled, fold in the cream until evenly mixed. Chill for 2 hrs.

To assemble, turn the cookies upside-down, matching the most similar-sized cookies into pairs. (The recipe should make six sandwiches, with a generous amount of mousse filling – you can serve any leftover mousse in cups with crushed meringues for a dessert.) Pipe or spread a thick layer (about 2-3cm thick) of mousse onto six cookies. Chill for 1 hr, then top with the other cookies and use a palette knife to spread the mousse smoothly around the middle.

Put the cookie sandwiches on a large plate and chill for at least 4 hrs, or overnight. Serve with the quartered strawberries and some ice cream on the side.