Warm up the milk and the heavy cream in a sauce pot. While its getting hot, in a bowl whisk well the egg yolks and the sugar until frothy and pale in color. Once the milk is hot, while constantly whisking slowly pour the milk into the eggs, then once all is mixed, pour the sauce back into the sauce pan and add the vanilla bean paste. Constantly stir on low to medium low heat until the sauce starts to thicken, it will take about 5-7 minutes. You want the sauce to coat the back of the spoon nicely. Be very careful once you see it starting to thicken, it goes from beautiful sauce to scrambled eggs fast if you aren't careful. Yes. my first batch was scrambled eggs because I got a phone call and walked away for half a minute, ha! It will also slightly thicken as it cools so beware of that if you're making it in advance. I don't like making it too much in advance because I like it served slightly warm over the Germknödel, but I've successfully reheated leftovers in the microwave.