Melt butter in a medium saucepan over medium heat. Add shallots; cook until soft and translucent, about 2 minutes. Add ginger and curry powder; cook, stirring, 1 minute. Add apples, potato, salt, and chicken stock. Bring to a simmer over medium-high heat, and cook until potato is tender when pierced with a paring knife, about 12 minutes. Remove from heat, and let cool slightly.
Use an immersion blender, or transfer mixture to the jar of a blender, working in batches if necessary so as not to fill more than halfway; puree just until smooth (do not overprocess). Return soup to pan; stir in cream, and season with salt and pepper. Place over medium heat until soup is just heated through; do not let it boil. Divide soup among serving bowls, and garnish with sour cream.