Heat a large saucepan over medium-high heat. Coat pan with cooking spray. Add onion to pan; sauté 5 minutes. Stir in cumin and annatto; sauté 30 seconds.
Combine 2 tablespoons milk and egg yolk. Chill. Add remaining milk, potato, cilantro, and jalapeño to pan; bring to a boil. Cover, reduce heat, and simmer 35 minutes, stirring occasionally. Partially mash potato mixture with potato masher.
Stir 1/4 cup potato mixture into egg yolk mixture. Return potato mixture to pan; stir in salt and black pepper. Simmer 5 minutes, stirring frequently. Remove egg yolk from 2 hard-cooked eggs; coarsely chop egg whites and whole hard-cooked egg. Ladle about 1 cup soup into each of 6 bowls; top each serving with 2 teaspoons chopped egg mixture, about 1 tablespoon avocado, 2 teaspoons cilantro, and 1/4 teaspoon jalapeño. Garnish each serving with hot pepper sauce, if desired.
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