Lightly butter a standard 12-cup muffin tin, and set aside. Stir together yeast, water, and sugar in a small bowl, and let stand until foamy, about 5 minutes. Stir until yeast dissolves.
Whisk together flours and salt in a large bowl. Using a pastry blender, cut in butter pieces until mixture resembles coarse meal. Stir in yeast mixture. Stir in sour cream and thyme until combined.
Spoon a heaping 1/3 cup dough into each cup of muffin tin. Cover with buttered plastic wrap; let rise in a warm place until doubled in size, about 1 hour.
Preheat oven to 400 degrees. Bake rolls 10 minutes. Remove from oven, and lightly brush with 2 tablespoons melted butter. Bake until golden brown, 16 to 18 minutes. Immediately invert and unmold rolls onto a wire rack. Serve warm.