This filling acts as a glue to hold the layers together and works best when chilled, so don't skip that step.
Stir together cornstarch, almond extract, and 2 Tbsp. water in a small bowl.
Bring whipping cream, brown sugar, and butter to a boil in a saucepan over medium heat. Remove from heat, and immediately whisk in cornstarch mixture. Stir in coconut and sour cream. Cover and chill 8 hours.