1. Prepare Muffins: Preheat oven to 350°. Beat butter at medium speed with a heavy-duty electric stand mixer until creamy. Gradually add granulated sugar, beating until light and fluffy. Add egg yolks, 1 at a time, beating just until blended after each addition.
2. Stir together flour and next 2 ingredients. Stir together milk and next 3 ingredients. Add flour mixture to butter mixture alternately with milk mixture, beginning and ending with flour mixture. Beat at low speed just until blended after each addition. Beat egg whites until stiff peaks form; fold into batter. Stir in poppy seeds. Spoon into lightly greased 12-cup muffin pans, filling three-fourths full.
3. Bake at 350° for 18 to 20 minutes or until a wooden pick inserted in center comes out clean. Cool in pans on wire racks 5 minutes; transfer to wire racks.
4. Prepare Glaze: Stir together powdered sugar and orange juice. Drizzle over warm muffins.