Put the egg yolks in a mixing bowl and lightly whisk, then add the oil, a short drizzle at a time while continuing to whisk. This technique is important because the oil and egg will otherwise not emulsify. Mix in the garlic, mustard and vinegar, if using, and 1/4 tsp salt. Taste and adjust the seasoning if needed. Will keep chilled in a jar for up to three days.