Whisk together the heavy cream, whole milk, dark brown sugar and pinch of salt in a medium saucepan. Bring to a simmer over medium heat.
1
Whisk egg yolks, vanilla and cinnamon until it thickens for about three minutes. Add a scoop of the warm cream and whisk. Repeat several times.
2
Add egg-cream mixture back to the pot. Whisk and bring back to a simmer. Cook until mixture thickens and covers the back of a wooden spoon. Turn off heat.
3
Pour into a large bowl, and then directly cover the top of the custard's surface with plastic wrap and refrigerate until cold, at least four hours to ensure proper churning.
4
Pour custard in an ice cream maker and churn according to manufacturer's directions. In the last couple minutes of churning, add the crushed Biscoff cookies to the ice cream machine.
5
Pour the churned ice cream into an airtight, freezer-safe container. Freeze until firm for several hours before serving.
6