Start by preheating your oven to 350°F (175°C). Grease a 9x9-inch baking dish or line it with parchment paper. This prevents sticking and makes cleanup a breeze.

In a medium bowl, whisk together the flour, sea salt, and espresso powder. Then, in a saucepan over low heat, melt the butter and both sugars, stirring constantly until combined and smooth. Remove it from heat and mix in the cocoa powder. Let it cool slightly before adding the eggs and vanilla extract. Stir until well combined, and then slowly incorporate the dry ingredients. Finally, fold in the dark chocolate chips.

In another bowl, beat together the softened cream cheese and caster sugar until smooth. Then add the egg, vanilla bean paste, and lemon zest. Mix until everything is well combined and creamy. The lemon zest will add a delightful brightness.

Pour about two-thirds of the brownie batter into the prepared baking dish, spreading it evenly. Next, dollop spoonfuls of the cream cheese mixture over the brownie layer. Use the remaining brownie batter to create a final layer on top.

Using a knife or a skewer, gently swirl the cheesecake mixture into the brownie batter. Be careful not to over-mix; you want to create a marbled effect, not a blended one.

Place the baking dish in the oven and bake for 30-35 minutes. Check for doneness by inserting a toothpick; it should come out with a few moist crumbs, not wet batter. Once done, let it cool in the pan for a bit before transferring it to a wire rack.