Combine strawberries in a bowl with sugar, lemon zest, lemon juice, and vanilla.
Place mixture in a heavy bottom saucepan over medium-low heat, stirring until sugar has dissolved.
Raise heat to medium-high and cook until it reaches a temperature of 215 degrees on a candy thermometer, stirring to prevent scorching. If you don’t have a candy thermometer, place a plate in the freezer.
Once the plate has chilled well, place a small spoon of mixture on the plate to check the consistency.