01 -
Preheat the oven to 175°C (350°F) and spray or grease a 9-inch square baking pan.
02 -
In a large bowl, whisk together the flour, sugar, and baking powder. Cut the butter into small pieces and add to the mixture.
03 -
Add the egg, milk, softened butter, and lemon peel to the dry ingredients. Mix until just moistened. Finish mixing with your hands if needed to fully incorporate the flour. The batter will be very thick.
04 -
Fold in the blueberries. The batter's thickness may require extra effort. Spread the batter evenly into the prepared baking pan.
05 -
In a mini food processor or bowl, combine the sugar, flour, walnuts, and cinnamon. Add the cold butter and pulse (or cut in by hand) until the mixture is crumbly. Avoid over-processing. Sprinkle the topping evenly over the batter.
06 -
Bake in the preheated oven for 40-45 minutes, or until a toothpick inserted in the center comes out clean.
07 -
Combine the powdered sugar and milk in a small bowl. Adjust the consistency by adding milk a few drops at a time if it's too thick. Drizzle over the cooled cake, letting it set until the sugar solidifies.