1. Place potatoes in a saucepan, and cover with cold water to 2 inches above. Add 1/4 teaspoon salt; bring to a boil. Reduce heat, and simmer 15 minutes or until tender; drain. Return the potatoes to pan over low heat; add remaining 1 teaspoon salt and butter to pan. Mash the potatoes with a potato masher to desired consistency.
2. Add cheese, cumin, and chile powder to potato mixture; stir until cheese melts. Remove from heat.
3. Combine half-and-half, crushed garlic cloves, and chopped serrano chiles in a small saucepan; cook over medium-high heat to 180° or until tiny bubbles form around edge (do not boil). Remove from heat; let stand 10 minutes. Strain mixture through a sieve over bowl; discard solids. Stir half-and-half into potato mixture; cook over low heat 30 seconds or until thoroughly heated, stirring frequently. Remove from heat; stir in sour cream.