1. Preheat your oven to 350 degrees F. Line two (8x8 inch) square baking pans with parchment paper.
2. In a medium-sized, microwave-safe mixing bowl, combine butter and chocolate. Microwave on high for 30-second intervals, stirring after each interval until melted and smooth.
3. Whisk in granulated sugar, vanilla extract, instant coffee granules, and eggs until smooth.
4. Stir in the cocoa powder, flour, and kosher salt until just combined.
5. Divide the batter evenly between the prepared pans, spreading it into an even layer.
6. Transfer the pans to the preheated oven and bake for 10 to 12 minutes.
7. Once cooled, spread Nutella evenly onto each brownie layer.
8. Add mascarpone cheese and sweetened condensed milk to the bowl of a stand mixer. Whip until smooth.
9. Pour in the heavy cream, vanilla, and a pinch of flaky sea salt. Whip until stiff peaks form.
10. Spread the whipped ice cream in an even layer over the Nutella side of one of the brownie pans.
11. Gently place the other brownie on top of the ice cream, Nutella side facing down.
12. Cover the pan with plastic wrap and freeze for 4 to 6 hours.
13. Once frozen, slice the bars into rectangles.
