Place the egg yolks, cornstarch, and sugar in a medium bowl, whisk until completely combined and homogenous
Place the heavy cream and milk into a medium saucepan, heat the mixture on medium heat until its simmering
Making sure your bowl is secure on the counter (place a wet towel underneath) pour half of the hot milk mixture SLOWLY while whisking constantly into the egg mixture, this is tempering the egg
Take the tempered egg mixture and pour it back into the milk mixture on the stove, whisking constantly
Keep whisking on the stove until it has thickened and its bubbling, at this point you can take it off the heat and add the butter, extracts, and salt
Whisk until combined and the butter is incorporated, pour into a bowl, place plastic wrap directly on the surface so it doesn't form a skin
Place in the refrigerator until cold