Place the yolk in a stainless steel bowl. Begin whisking in the grapeseed oil drop by drop. Once the mixture has thickened and emulsified, you can whisk in the remaining grapeseed and olive oils in a slow steady stream. If the mixture gets too thick, add a drop or two of water.
Pound the garlic with 1/4 teaspoon salt with a mortar and pestle. Whisk the garlic paste into the aïoli. Season with 1/4 teaspoon salt, a squeeze of lemon juice, and the cayenne. Taste for balance and seasoning. If the aïoli seems thick and gloppy, thin it with a little water. In addition to thinning the aïoli, this will also make it creamier.
