Whisk together first 5 ingredients in a large, heavy saucepan. Cook over medium heat, stirring often, 4 minutes, or until sugar dissolves and mixture is hot. Gradually whisk about 1 cup hot milk mixture into yolks. Whisk yolk mixture into remaining milk mixture. Cook over medium heat, whisking constantly, 8 to 10 minutes or until mixture thickens and coats a spoon.
Pour through a fine wire-mesh strainer into a bowl, discarding solids. Cool completely. Chill 8 to 24 hours or until very cold.
Pour mixture into bowl of an electric ice-cream maker, and process according to manufacturer’s instructions. Freeze 1 hour before serving. Store up to 1 week.
In a medium bowl, whisk together yogurt, sour cream and lime juice. Add cilantro, season with salt and pepper and set aside.
In a large skillet over medium heat, heat oil. Add chicken and season with salt, pepper, cumin and chili powder. Cook until golden and no longer pink, 8 minutes per side. Transfer to a plate to let rest, then slice into cubes and let cool completely.
In a large bowl, combine cooked chicken with avocado, beans, corn and red bell pepper. Toss with yogurt dressing until completely coated. Serve on bread, in lettuce leaves, or by itself.