For the stout reduction in a small saucepan over medium high heat boil down the Guinness to 1cup of liquid concentrate. Don't overboil.
For the brownie in a another small saucepan melt together the butter and the chocolate. Take off heat, whisk in all of the brownie ingredients and 1/3-1/2cup of Guinness reduction (depending on taste) just until incorporated. Spread the batter into an 8x8 pan lined with parchment paper or aluminum foil lightly greased with cooking spray and bake in the oven, preheated to 170C for 30-35min. Take out, cool completely, cut 100gr brownie slice into bite-sized cubes, leave for later use.
For the chocolate stout sauce in a small saucepan over medium heat combine the leftover stout reduction, 1/2cup sugar, 1/3cup cocoa powder, cook 2min stirring until uniform. Add 30gr chopped dark chocolate, cook until completely melted and glossy.
Dissolve the cornstarch in 50ml whole milk, leave to rest.
For the ice cream in another small saucepan over medium heat combine the heavy cream, the leftover whole milk, 2/3cup sugar, pinch of salt, stir, bring to an almost boil, add the cornstarch milk and cook constantly stirring until thickened. Take off heat, add in the cream cheese and 1/3-1/2cup chocolate stout sauce (depending on taste), stir until smooth and combined. Cool to room temperature, then transfer into the fridge overnight.
Freeze in your ice cream maker according to the manufacturer's instructions adding the beer brownie cubes at the last couple of minutes of churning. Store in an airtight container or spoon right away.