Preheat the oven to 350 F and spray or grease a 9-inch square baking pan.
In a large bowl, whisk together flour, sugar and baking powder. Cut butter into small pieces. Add egg, milk, butter and lemon peel; mix just until dry ingredients are moistened. You may need to finish mixing with your hands to get all the flour incorporated. the batter will be very thick.
Fold in the blueberries. Because the batter is so thick, this may take a few minutes. Spread into a greased 9-in. square baking pan.
For topping, combine sugar, flour, walnuts and cinnamon in a mini food processor or bowl. Add butter and process, or cut in if doing by hand, until mixture is crumbly. I like to do this with my fingers so I can tell when it's ready by touch. Sprinkle over batter. Bake at 350 degrees F for 40-45 minutes or until cake tests done.
For drizzle, combine the powdered sugar and milk. If too thick, add a few drops of additional milk at a time until it reaches drizzling consistency. Drizzles over the top of the cake and allow to sit until sugar solidifies.