The consistency of homemade mayonnaise will be different from that of store bought. When you make it with a whisk, it will be loose and almost pourable. If you use a blender or food processor, it will be firmer because generally more air is whipped into it.

Preheat oven to 200°F and place a rack in the center. Set a wire rack on top of a rimmed baking sheet and place next to the stovetop.

To prepare potatoes: Soak cut potatoes in cold water for 30 minutes. To an electric deep fryer or large Dutch oven set over medium heat, add oil. Heat oil to 330°F.

To prepare mayonnaise: To a medium mixing bowl add egg yolk, garlic, and water; whisk together. To a small liquid measuring cup add olive oil and vegetable oil, stir to combine. While whisking the egg mixture vigorously, add oil in drops to form an emulsion. Continue whisking and slowly drizzling in the oil to form a thick sauce. Taste and adjust seasoning with lemon juice, black truffle salt and pepper. Cover and keep refrigerated until ready to use.

To cook potatoes: Drain potatoes and dry very thoroughly with paper towels. Working in batches, blanch potatoes in hot oil until nearly cooked through but not browned, 5 to 6 minutes, adjusting the heat of the stove to maintain oil temperature close to 330°F. Remove blanched potatoes from oil with a slotted spoon or spider and place on the cooling rack. Repeat process with remaining potatoes.

Increase heat so oil temperature registers 375°F. Working in batches, fry blanched potatoes until crispy and golden brown, 3 to 5 minutes. Remove fried potatoes from oil with a slotted spoon or spider, draining excess oil. Place back on the wire rack next to stovetop and salt immediately. Place rack and baking sheet in preheated oven to keep fries warm. Repeat with remaining fries and serve with truffle mayonnaise.