Whisk egg yolk, vinegar, and 1 TBSP water in a small bowl.

Bring 2 inches of water to a simmer over medium low heat.

Set the bowl with the yolk mixture over the sauce pan. Slowly whisk until a thermometer registers 150 degrees.

Set the bowl in ice water to stop the cooking. Whisk until the mixture is lukewarm.

Put the bowl on a towel. Add 1/4 TSP salt. Combine the oils. Whisking constantly, slowly drizzle the oil into the bowl until smooth and thick. Season with salt. Cover and store in refrigerator up to 4 days.