Mix dry ingredients. Add cornstarch and sea salt to a small mixing bowl. Mix the two ingredients until there are no lumps. Set aside.
Mix wet ingredients. Add the milk, maple syrup, and egg yolks to a medium-sized mixing bowl. Whisk the mixture until all ingredients are fully incorporated and the egg yolks are completely broken up.
Combine. Slowly add the dry mixture to the milk mixture. Whisk as you combine the two mixtures until it has a smooth consistency and all ingredients fully dissolve.
Heat mixture. Pour the mixture into a medium-sized saucepan and set it over medium heat. Continue to whisk the mixture as it heats up to keep it from overcooking and burning. After 4-5 minutes, the mixture will begin to thicken. Once thickened, remove from the heat.
Add butter and vanilla. Add butter and vanilla to the thickened pudding and whisk once more until everything is blended and your pudding has a nice, smooth, thick consistency.
Cover pudding. Pour the pudding into a bowl or container. Place a sheet of plastic wrap over the pudding, directly touching the surface of the pudding, and secure it around the sides of the bowl. This will help prevent any skin from forming as it cools.
Chill. Place the covered pudding in the refrigerator for at least one hour to chill before enjoying.