1 Lightly grease and line a 20cm (8in) square cake tin, leaving a slight overhang of paper. This will allow you to remove the slab easily once cooked through.
2 Start by making the shortbread. Mix the flour, cornflour and salt together using a fork.
3 Using a stand mixer or electric hand mixer, cream the butter, sugar and vanilla until pale and fluffy, or cream by hand with a wooden spoon. Tip in the flour and mix until a crumbly dough is formed. Knead the mixture together to form a dough, then press into the base of the prepared tin, using the bottom of a glass to level the surface, then prick the base with a fork.
4 Preheat the air fryer to 160°C. Bake for 25–28 minutes, or until golden brown.
5 To make the caramel, heat all the ingredients together in a medium non-stick saucepan, on a low heat to start, stirring often and ensuring the sugar granules have dissolved completely and the mixture is smooth. Be patient as this can take up to 5 minutes to avoid a grainy caramel. Then bring to the boil on a low–medium heat and let bubble for 5–6 minutes. The caramel will become thickened and a little darker in colour. Pour over the biscuit base, level off and allow to cool and set completely.
6 To make the chocolate topping, melt the chocolate and butter in the microwave in 30-second bursts, stirring until smooth, or melt in a bain-marie. Pour over the set caramel, level off and bang the tin on a work surface to level out the top. Refrigerate to firm up and set. Cut into 16 squares.
ADAPTATION: Substitute the flour for a gluten-free blend
TIP: Halve the quantities and bake in a 450g (1lb) loaf tin for 18–20 minutes.