Both are custard based desserts, but there are a few differences in ingredients and preparation. The main difference is how they are topped. While flan has a soft caramel topping, creme brûlée has a sugar topping that is broiled or torched so it’s hard and crispy. (If you’re looking for creme brûlée, check out this sous vide creme brûlée or creme brûlée with lemon).