Preheat oven to 375°. Whisk together first 5 ingredients. In another bowl, whisk together eggs, melted butter and vanilla; add to flour mixture, stirring just until moistened (batter will be stiff). Fold in apples.
Fill 12 greased or paper-lined muffin cups three-fourths full. For topping, mix brown sugar, flour and cinnamon; cut in butter until crumbly. Sprinkle over batter.
Bake until a toothpick inserted in center comes out clean, 15-20 minutes. Cool 5 minutes before removing from pan to a wire rack to cool. Mix glaze ingredients; drizzle over tops.