For Streusel Topping, in medium bowl, stir together pancake mix, sugar and cinnamon. Cut in butter with pastry blender or two knives, until mixture is combined and crumbly; set aside.
For Coffee Cake, in large bowl, beat butter on medium speed of mixer until smooth, then add sugar and continue beating until creamy. Add milk, egg and vanilla extract; beat until combined (mixture will appear curdled). Add pancake mix, mixing on low speed until combined and smooth, scraping bowl as necessary.
Spray 9-inch square pan with cooking spray. Spoon batter into pan and spread evenly. Stand apple slices in batter. Sprinkle with Streusel Topping.
Bake 30 to 35 minutes or until pick inserted in center comes out clean. Cool on wire rack. Add Drizzle, if desired. Serve slightly warm or at room temperature.
Drizzle: Stir together 1/2 cup confectioners’ sugar and just enough water or milk for desired drizzling consistency. Drizzle over coffee cake.