In a small heavy-bottomed pot off the heat, whisk together the corn starch, brown sugar, and pinch of salt. Gradually whisk in the milk, making sure that the no lumps form. Whisk in the egg yolk, and the almond and vanilla extract. Put the pot over medium heat and gently heat for 10 to 15 minutes, continuously stirring until the mixture is thick and bubbling. Turn off the heat.
Peel and pit the avocado and mash or blend until very smooth. Stir into the warm custard, taste for sweetness, then transfer to a bowl, cover with plastic and allow to cool and set in the refrigerator for at least 1 hour.