Whisk together granulated sugar and egg yolks until creamy and pale, about 2 minutes.

In a saucepan, heat whole milk, heavy cream, sugar, kosher salt, and cinnamon over medium heat until simmering.

Gradually ladle some hot milk into the whisked egg yolks, whisking continuously to prevent curdling.

Return the tempered mixture to the saucepan and cook over low heat until thickened, about 8-10 minutes.

Stir in vanilla extract, then cool to room temperature and refrigerate for at least 4 hours.

Pour chilled custard into ice cream maker and churn according to instructions, adding crushed Biscoff cookies in the last minutes.

Transfer half of the churned ice cream to a loaf pan, swirl in cookie butter, then add remaining ice cream.

Cover with plastic wrap and freeze for at least 4 hours until firm.