In a saucepan over low heat, heat the milk, sugar, 1 cup of the heavy cream and the salt. Stir until the sugar is dissolved. Scrape the seeds from a vanilla bean, and add to the milk, add the rest of the bean as well. Cover. Remove from heat and let steep at room temperature for 30 minutes.
Pour 1 cup heavy cream into a large heat safe bowl. Place a fine-mesh sieve on top. In a separate medium sized bowl whisk the egg yolks. Slowly add the warm milk mixture to the eggs, whisking constantly. Pour back into the saucepan. Return the saucepan to the stove, and heat over medium-low. Using a rubber spatula stir constantly, until the mixture thickens to coat the back of the spatula (170-175 degrees). Pour the mixture the sieve into the bowl with the heavy cream. Add the vanilla extract and stir until well combined. Put the bean into the mixture as well. Cover and place in the fridge until cold. Can take up to 8 hours, over night works great.
Once milk mixture is cold churn in an ice cream maker following the manufacturer's instructions. Once finished transfer to an air tight container. Add crushed Biscoff cookies, stirring to combined. Heat Biscoff Spread in the microwave for about 20 seconds, just to get it easier to mix in. Using a spoon, drizzle a few spoonfuls of the spread over the ice cream. Gently fold it in. Repeat until all of the Biscoff is used. Place in the freezer for at least an hour before serving.