1. Preheat oven to 375°.
2. Weigh or lightly spoon flour into dry measuring cups; level with a knife. Combine flour and next 4 ingredients (through salt) in a large bowl, stirring with a whisk. Cut open tea bags; add tea to flour mixture, stirring well. Make a well in center of mixture. Combine buttermilk, butter, 1 teaspoon vanilla, and egg in a bowl, stirring well with a whisk. Add buttermilk mixture to flour mixture, stirring just until moist.
3. Place 12 muffin-cup liners in muffin cups; coat liners with cooking spray. Divide batter evenly among prepared muffin cups. Sprinkle nuts evenly over batter. Bake at 375° for 15 minutes or until a wooden pick inserted in center comes out clean. Cool for 5 minutes in pan on a wire rack.
4. Combine remaining 1/2 teaspoon vanilla, powdered sugar, and 1 tablespoon water, stirring until smooth. Drizzle evenly over muffins.