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.
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.
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.
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.
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.
Pour the churned ice cream into an airtight, freezer-safe container. Freeze until firm for several hours before serving.