Stir cumin in a small skillet over medium heat until fragrant, 2 minutes; let cool. Coarsely grind in a spice mill. Whisk yolks, lemon juice, and garlic in a small bowl. While whisking, slowly pour in oil drop by drop, whisking vigorously until emulsified. Whisk in cumin and 1/2 teaspoon water. Season with salt. Cover; chill.