In a large sauce combine guinness, cream, and milk. Bring mixture to JUST a boil, then remove from heat.
In a large bowl beat together egg yolks and brown sugar until light and fluffy. Add a small amount of the hot guinness mixture to the eggs and whisk it quickly together (to temper the eggs), then carefully pour the remaining guinness mixture into the egg mixture, whisking to incorporate.
Pour the mixture back into the saucepan and heat over medium-low - whisking constantly - until the mixtures thickens enough to coat the back of a spoon. Do NOT let the mixture come to a boil or it will curdle.
Once thickened remove pan from heat, and stir in vanilla. Pour the custard into a large heat proof bowl, cover with saran wrap touching down to the custard (to prevent skin from forming), and place bowl in the fridge for at least 3 hours.
When ready, pour custard in your ice cream maker and freeze according to your machines instructions.
Once it's finished churning, stir in chocolate chunks. Transfer ice cream to an airtight container and freeze for at least 6 hours.
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