In blender, place the tomatillos, onion, garlic, Serrano pepper, cilantro, chicken bouillon granules, 2 tablespoons water, sugar, and ½ teaspoon salt. Blend for about 2 minutes or until smooth.
In a 1-quart non-stick saucepan, heat 1 tablespoon oil over medium-high heat until hot. Pour in the tomatillo mixture and cook, stirring occasionally, for 3 to 5 minutes or until the salsa turns to a deeper green. Set aside.
Place the chorizo in a small non-stick skillet and cook for about 3 to 5 minutes, stirring constantly, until it is well cooked and crumby. Set aside.
In a small bowl, mix the sour cream and milk, stirring constantly for 1 or 2 minutes. Set aside.
Combine the masa flour, 1 ½ cup water and 1 teaspoon salt in a large bowl and mix by hand for about 1 to 3 minutes, or until the dough is moist throughout. Using wet hands, form the masa mixture into 8 equal flat disks of about 4 inches in diameter. Spoon 2 teaspoons of refried beans on the center, then fold both sides to enclose refried beans. Form 1/2 inch thick elongated tortilla patties or “tlacoyos” of about 4 inches in length.
In a 12-inch nonstick skillet, cook the tlacoyos over medium heat for 8 to 12 minutes, or until black spots appear on both sides, turning once. Brush the tlacoyos on both sides with the remaining canola oil and cook over medium-high heat for an additional 2 to 4 minutes until slightly fried. Place the tlacoyos on a platter. Top with chorizo, tomatillo salsa, and Cotija cheese. Drizzle with sour cream sauce. Serve immediately.