• Preheat the oven to 180°C/ 160°C (fan) 350°F. Grease a 20-cm (8-inch) square metal baking pan and line with enough parchment paper to allow for 2-cm (1-inch) wings on all sides. (If you only have a glass or ceramic pan, that’s okay; just prepare to bake these a little longer!)
• To make the crust, in a food processor, process the living daylights out of your digestive biscuits (graham crackers) and salt. With the processor running, drizzle in the melted butter and process for a few more seconds, until the mixture clumps together. Pour the mixture into the prepared pan and press it down firmly and evenly.
• Bake until just set; begin checking for doneness at 10 minutes.
• To make the brownie layer, whisk together the butter, sugar, cocoa, and espresso powder in a large bowl. Whisk in the egg yolks one at a time, then add the vanilla. Sprinkle the flour, salt, and baking powder evenly over the surface of the batter. Give the dry ingredients a rough little whisk to combine, then incorporate them into the rest of the batter. Pour onto the crust (it’s totally okay if the crust is still a little warm; you just don’t want it to be piping hot), spread it out evenly, and bake until just set in the centre; begin checking for doneness at 20 minutes. Let cool completely in the pan.
• To make the marshmallow layer, combine the egg whites, sugar, cream of tartar, and salt in the bowl of your stand mixer and set it over a pot of simmering water, making sure that the water doesn’t touch the bottom of the bowl. Heat, whisking continuously, until the mixture reaches 80°C (175°F) (you can either clip a candy thermometer on the side of the bowl or stick an instant- read thermometer in occasionally); it will be thin and frothy. Transfer the bowl to the stand mixer fitted with a whisk and beat on high until stiff and glossy, about 3 minutes, then mix in the vanilla. Spread over the brownies and then stick the pan under the broiler for a few minutes until it reaches your desired level of toastiness. Check it frequently because broilers work quickly! (You can also brown with a kitchen torch but remove the bars from the pan and take off the parchment first.) Let cool completely in the pan, then cut into 16 squares using a moist sharp knife.
• Store in an airtight container in the fridge for up to 4 days.

Recipe extracted from Sweet Farm (Cookies, Cakes, Salads (!), and Other Delights from My Kitchen on a Sugar Beet Farm (William Morrow)