With an electric mixer, beat the egg yolks and vanilla sugar until foamy. I did mine for about 2 – 3 minutes. You can kind of see the texture here.
8 large egg yolks, 8 grams vanilla sugar, 2 cups + 2 tablespoons (250 grams) powdered sugar
Slowly add the powdered sugar. It’ll look like this. Make sure there aren’t clumps of powdered sugar like I have below!
Then add the evaporated milk and then the rum. Mix slowly until well combined.
1 1/2 cup + 1 tablespoon (375 ml) evaporated milk, 1 cup + 1 tablespoon (250 ml) dark rum
Put it in a pot, but don't fill it all the way to the top. Leave at least 1" at the top of the pot. If you have to do it in two batches, then do it in two batches. If you fill it almost to the top of the pot, it will likely come out badly. Then put that pot in a larger pot, which has been filled slightly less than halfway with water. How far you’ll need to fill it depends on the size of the pots. Heat it over medium heat slowly until it reaches 160 °F (71 °C), stirring occasionally. Make sure to scrape the bottom and sides of the pot every now and then. It normally takes 15 – 20 minutes. Do not let the water or liqueur simmer. If you let it go above 185 °F (85 °C) it’ll become scrambled eggs. So be really careful and stop cooking at 160 °F (71 °C). Add your vanilla now if you didn't use vanilla sugar. The liqueur should be quite thick now. Remove the pot from the heat.
To ensure your bottles don't crack, run some hot water over the outside of your bottles and then dry them off.
Use a funnel to pour the liqueur into the bottles, filling all the way up to about 1" from the top.
Let the bottles cool completely and then store in the refrigerator for up to 3 months.