Heat Milk Mixture. Combine milk, maple syrup, vanilla, and salt in a pot. Whisk until smooth and bring to a boil, stirring occasionally.
2 cups milk, 1/3 cup maple syrup, 2 teaspoons vanilla extract, 1/8 teaspoon salt
Prep Yolks and Cornstarch. While the milk heats, whisk yolks, cornstarch, and 2 tablespoons of the milk mixture (while it's still lukewarm) in a bowl.
4 egg yolks, 4 tablespoons cornstarch
Temper Eggs. Gradually whisk hot milk into yolk mixture in small additions to avoid curdling.
Thicken Cream. Pour the mixture back into the pot and cook over medium heat, stirring constantly, until thick and bubbly (2-4 minutes).
Whisk Off Heat. Remove from heat and whisk for about a minute until the cream is smooth.
Add Butter. Stir in butter until fully incorporated.
2 tablespoons unsalted butter
Adjust. Taste and adjust salt or sweetness (by adding regular sugar) if needed.
Strain. Strain if there are any lumps.
Chill. Cover with plastic wrap touching the surface to prevent a skin. Cool at room temperature, then refrigerate.
Smooth and Use. After chilling, whisk until smooth and it’s ready to be piped.