In small saucepan, whisk together egg yolks, cream, milk and sugar. Heat mixture over medium-medium high heat and continue stirring until mixture thickens and coats the back of a wooden spoon. Swipe your finger across the mixture on the back of the spoon and if the edges stay put, it’s ready. Takes about 5 minutes, more if your temperature is lower. If you have a candy thermometer cook until 170º.
Strain mixture into bowl over fine mesh strainer to remove any bits of egg that “scrambled”. Set bowl in ice bath for about 30 minutes to cool down (stir occasionally). Pour ice cream base into Ninja Creami pints if using your Ninja Creami ice cream maker. Alternatively, store custard base in refrigerator for a few hours or overnight to thoroughly chill prior to churning with your ice cream maker.
For Ninja Creami ice cream maker machine: Spin pint on ice cream setting. Then add a scoop of cookie butter and crushed cookies and spin on the mix-in setting.
For other Ice Cream makers: Pour custard base in your ice cream maker and process according to your ice cream maker‘s instructions. For me, it’s 20 minutes. Remove churned ice cream to a freezer container, swirl in cookie butter and crushed cookies and let freeze several hours or overnight to firm up.
To serve, heat a little biscoff cookie spread in the microwave to make it runny and drizzle over ice cream, top with crushed biscoff cookies, if desired.