Line a baking sheet with parchment paper or a silicone mat.

In a medium saucepan, melt butter over medium heat. Stir in sugar, milk, salt, and lemon zest. Cook, stirring constantly, until sugar dissolves and mixture boils gently.

Remove from heat. Stir in lemon juice and vanilla extract.

Quickly fold in oats until coated.

Using a cookie scoop or spoons, drop heaping tablespoons of mixture onto the prepared sheet, spacing 2 inches apart.

Refrigerate for 30 minutes until cookies are firm.

Let rest 5 minutes at room temperature before serving.