Beat butter at medium speed with a heavy-duty mixer until creamy. Add 2 1/2 cups sugar, beating 4 to 5 minutes or until fluffy. Add egg yolks, one at a time, beating just until yellow disappears.
Combine flour and baking soda; add to butter mixture alternately with sour cream, beginning and ending with flour mixture. Stir in flavorings.
Beat egg whites until foamy; gradually add 1/2 cup sugar, 1 tablespoon at a time, beating until stiff peaks form. Fold into batter.
Pour batter into a greased and floured 10-inch tube pan.
Bake at 325° for 1 1/2 hours or until a long wooden pick inserted in center comes out clean. Cool in pan 10 to 15 minutes; remove from pan, and place on a serving plate. While warm, prick cake surface at 1-inch intervals with a wooden pick; pour warm Buttered Rum Glaze over cake. Let stand 4 hours or overnight before serving.