Cookie s’mores traybake

Method

1. Heat the oven to 180°C/160°C fan/ gas 4. In a large bowl, mix the melted butter and both sugars using a wooden spoon or electric mixer until combined (the mixture will have a grainy texture). Set aside to cool for 10 minutes.
2. Beat in the egg and vanilla bean paste, mix in the plain flour, bicarbonate of soda and cornflour, then gently stir in the chocolate chips. Spread the mixture evenly into the prepared tin, then press the butter biscuits in a neat layer over the top. Bake for 15 minutes, then remove from the oven and leave to cool completely in the tin (see Make Ahead).
3. Meanwhile, make the marshmallow topping. Put the egg whites and caster sugar in a heatproof bowl and gently whisk with an electric hand mixer until frothy. Set over a pan of just simmering water (make sure the bowl doesn’t touch the water) and continue to whisk for about 10 minutes until the sugar has dissolved and the meringue is stiff and glossy. Add the marshmallows to the bowl and whisk on a low speed until they have melted completely. Remove the bowl from the heat and whisk for another 5 minutes until the mixture has slightly cooled.
4. Spread the marshmallow mixture over the top of the cooled biscuit base in pretty swirls. Use a kitchen blowtorch to lightly scorch the mallow topping (or pop under a hot grill for a minute, watching it like a hawk). Leave to cool fully in the tin, then serve in slices.