Prepare Your Baking Tin: Preheat your oven to 175°C (160°C fan/350°F). Line a 20cm x 20cm (8-inch x 8-inch) square baking tin with parchment paper, leaving an overhang on two sides to help lift the brownies out later. You’ll find the paper sits smoothly and prevents sticking.

Begin the Brownie Batter: In a large mixing bowl, cream together the softened unsalted butter and 100g granulated erythritol until the mixture is light and fluffy, taking on a pale, creamy colour. This should take about 3-4 minutes with a hand mixer. Scrape down the sides of the bowl to ensure everything is incorporated.

Add Wet Ingredients to Brownie: Beat in the two large eggs one at a time, ensuring each is fully incorporated before adding the next. Then, stir in the 1 teaspoon of vanilla extract until the mixture smells wonderfully fragrant and looks smooth and glossy.

Combine Dry Ingredients for Brownie: In a separate medium bowl, whisk together the unsweetened cocoa powder, almond flour, baking powder, and salt. Gradually add this dry mixture to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix; stop as soon as you no longer see streaks of dry ingredients. The batter will be thick and fudgy.

Prepare the Cheesecake Swirl: In a clean bowl, beat the softened full-fat cream cheese with 50g granulated erythritol until smooth and creamy, without any lumps. Add the egg yolk and ½ teaspoon vanilla extract, beating until just combined and the mixture is light yellow and silky.

Assemble and Swirl: Pour about two-thirds of the brownie batter into the prepared baking tin and spread it evenly with a spatula. Drop spoonfuls of the cheesecake mixture over the brownie layer, then add the remaining brownie batter in dollops over the cheesecake. Using a knife or a skewer, gently swirl the batters together to create a beautiful marbled pattern. Don't overmix, or you'll lose the distinct layers; aim for a captivating swirl.

Bake to Perfection: Place the tin in the preheated oven and bake for 30-35 minutes. The brownies are ready when the edges are set and a skewer inserted into the brownie portion comes out with moist crumbs, but not wet batter. The cheesecake swirl will look slightly golden at the edges and set in the centre. You'll notice a delightful chocolatey aroma filling your kitchen.

Cool and Slice: Allow the brownies to cool completely in the tin on a wire rack. This is crucial for achieving that signature fudgy texture and clean slices. Once completely cool (which might take an hour or two, or even longer if you're patient!), lift them out using the parchment paper overhang and slice into 16 squares. The brownies will feel firm but yielding to the touch.