Very Berry Croissants

Name: Very Berry Croissants
Ingredients: 8 tablespoons salted butter, softened; 1 cup almond paste; 1/2 cup dark rum; 1 tablespoon all-purpose flour; 2 teaspoons almond extract; 1 large egg; 1 1/2 sticks salted butter, cold; 1 1/2 cups all-purpose flour; 1 1/2 cups old-fashioned oats; 1 cup turbinado sugar; 3/4 cup brown sugar; 1 teaspoon ground cinnamon; 1 teaspoon kosher salt; Six 1-day-old store-bought croissants; 1/4 cup simple syrup, homemade (recipe follows) or store-bought; 3 cups frozen blueberries; Powdered sugar, for dusting; 1 cup granulated sugar