Beat 1/2 cup butter and shortening at medium speed with an electric mixer until creamy; gradually add 1 cup sugar, beating well. Add eggs, 1 at a time, beating until blended after each addition.
Combine 2 cups flour, baking powder, and salt; add to butter mixture alternately with milk, beginning and ending with flour mixture. Beat at low speed until blended after each addition. Stir in flavorings, 1/2 cup macadamia nuts, and 1/2 cup white chocolate morsels. Pour batter into a greased and floured 9" x 3" springform pan.
Combine remaining 1/2 cup macadamia nuts, brown sugar, oats, 1/4 cup butter, and remaining 1/4 cup flour; stir until crumbly. Sprinkle oat mixture over batter.
Bake at 350° for 50 to 60 minutes or until a wooden pick inserted in center comes out clean. Cool in pan on a wire rack 10 minutes. Run a knife between cake and pan to release cake; remove sides of pan, and cool on wire rack.
Place remaining 1/2 cup white chocolate morsels in a small heavy-duty zip-top plastic bag. Seal bag. Dip bag in very hot water 2 to 3 minutes or until morsels melt. Remove bag from water; snip a tiny hole in 1 corner of bag. Drizzle white chocolate over coffee cake.