In a bowl or large measuring cup, whisk the egg into the milk until fully combined. Let’s add our vanilla and salt and set it aside until you know your butterscotch mixture has cooled enough to begin tempering.
Slowly add a little of the hot butterscotch mix, whisking or stirring the whole time so it “tempers” the milky egg mixture. This will bring the milk temp up a little, so when we pour it back into the hot pan, it won’t scramble the eggs. Then, slowly pour the whole milk/egg mix into the saucepan with your butterscotch mixture, whisking continuously.
Cook over medium-low heat, stirring often, until it just begins to steam and thicken (don’t boil).
Stir in the oats and continue to cook for 3–5 minutes, stirring frequently, until thickened and creamy then take off the heat and cover for another 5 or so to thicken fully. Top with the crunchy oat crumble and enjoy!