Cherry nanaimo bars

Line a 20cm x 25cm baking tin with foil, covering the sides. Blitz the biscuits in a food processor to a fine rubble texture, then put in a bowl with the walnuts.

Melt 150g of the butter and the mahlab in a heatproof bowl over a pan of simmering water, making sure the bowl doesn’t touch the water’s surface. Add the caster sugar and cocoa powder, still on the heat, and whisk well until fully combined.

Remove the bowl from the heat and, whisking continuously, gradually pour in the egg. Return the bowl to sit on the pan of simmering water and heat for 1 min, whisking vigorously, until thickened. Pour the buttery-chocolate mixture over the biscuit crumbs and mix well. Pack the mixture into the base of the lined tin, patting down with your hands, into an even layer. Leave to refrigerate for 30 mins.

For the cherry-custard icing, use a stand mixer with a paddle attachment to beat 200g of the butter until smooth. Next, add the icing sugar, custard powder and double cream, beating slowly, before beating in the chopped cherries and syrup. Spread over the chilled biscuit base in a smooth thick layer. Leave to refrigerate for 2 hrs.

For the topping, melt the chocolate in a clean bowl over a pan of simmering water or in the microwave, then mix through the remaining butter until well combined. Spread over the custard layer and leave to refrigerate for another 2 hrs. Slice into 12 square bars with a warm knife to serve.