Preheat oven to 325°F. Line a rimmed baking sheet with parchment paper.
Whisk egg white and 1 tablespoon water in a large bowl until foamy; stir in melted butter and vanilla if using.
Add 4 cups mixed raw nuts to bowl and toss until evenly coated with the egg-white mixture.
Mix 3 tablespoons sugar, 2 teaspoons cinnamon, and 1 teaspoon fine sea salt in a small bowl; sprinkle over nuts and toss until sugar evenly clings.
Spread nuts in a single layer on prepared sheet. Bake 20 minutes, then stir and spread again; bake another 8–10 minutes until sugar is dry, slightly crackled, and nuts are fragrant and lightly browned at edges.
Immediately sprinkle flaky sea salt if desired and separate any clumps with a spatula; cool completely on the sheet until sugar is crisp (about 30 minutes).