Prep the berries. If using fresh strawberries, rinse, hull, and slice. If using frozen, no need to thaw.
Cook the syrup. In a small saucepan, combine strawberries, sugar, water, and a pinch of salt. Bring to a gentle simmer over medium heat, then reduce to medium-low.
Cook for 8–10 minutes, stirring and mashing the berries lightly as they soften.
Finish the flavor. Remove from heat. Stir in lemon juice and vanilla, if using. Taste and adjust sweetness or acidity.
Strain for smoothness. Set a fine-mesh strainer over a bowl or measuring cup.
Pour the mixture through and press gently with a spoon to extract the syrup. Discard the pulp or save it for yogurt or oatmeal.
Chill the syrup. Let it cool to room temperature, then refrigerate until cold. Cold syrup blends better with cold milk.
Mix your milk. For each serving, add 2–3 tablespoons of syrup to 1 cup of cold milk.
Stir well or shake in a jar until fully pink and frothy.
Serve. Pour over ice if you like it extra cold. Taste and add more syrup if you want a stronger strawberry flavor.