In the work bowl of a food processor or the container of a blender, puree strawberries until smooth.

In a heavy saucepan, combine sugar and cornstarch. Add strawberry puree, lemon juice, and egg yolks. Cook over medium heat, whisking constantly, for 7 to 8 minutes or until very thick. Remove from heat; gradually whisk in butter until melted. Stir in strawberry extract. Let mixture cool slightly; cover, and refrigerate for 2 hours.