In a small microwave-safe bowl, combine crème fraîche and cream. Heat on high until an instant-read thermometer registers 100°F (38°C), about 20 seconds.
In a medium saucepan, heat sugar and 3 tablespoons (45 grams) water over high heat, being careful not to splash sides of pan. (It should be the consistency of wet sand.) Use remaining 1 tablespoon (15 grams) water to brush down sides of pan, and gently whisk to help sugar dissolve. (Do not stir once it starts to boil.) Cook until desired light amber color is reached. Remove from heat; slowly add warm crème fraîche mixture, whisking to combine. Add butter, a few pieces at a time, whisking until combined. Stir in vanilla. Let cool completely.
Recipe by Bake from Scratch at https://bakefromscratch.com/persimmon-apple-crisp/