In a large bowl, whisk the egg yolk. Add a drizzle of olive oil, and whisk vigorously until mixture is nice and thick. Continue drizzling and whisking until 1/4 cup of olive oil's been added. Mixture should be pale with soft peaks. Next, add olive oil more quickly – 1 - 2 tablespoons at a time, until it's all been incorporated. Finally, whisk in the lemon juice, then the vinegar and sea salt. Taste, and adjust seasonings as needed.