Drape a kitchen towel over a small saucepan; set a small metal bowl over (this will hold bowl in place). Whisk egg yolk*, garlic, 1/4 teaspoon salt, and 2 teaspoons water in metal bowl to blend well. Whisking constantly, slowly drizzle in grapeseed oil, 1 teaspoonful at a time, until sauce is thickened and emulsified. Whisking constantly, add olive oil in a slow, steady stream. Stir in cayenne; season aioli with lemon juice, pepper, and salt.
