In a heavy saucepan, dissolve cornstarch into water on medium high heat, whisking until thick, about 2 minutes
Add rest of ingredients (not stout) to pan, lowering temp to medium heat. Stir until sugar is melted, should take about 2-3 minutes
Let cool for about 20 minutes, add stout (if using an ice cream maker, pour contents and churn at this point)
If using a freezer bowl only, pour into bowl and let freeze for 45 minutes. Stir mixture and let sit another 30 minutes
Make milkshake: You can blend with some ice and a little milk for a thicker milkshake, or you can scoop into cups and stir until soft. Drink and enjoy!