Preheat the oven to 375°F and grease an 8x8-inch square baking pan with cooking spray and set aside.
In a large bowl, whisk together the flour, sugar, salt, and baking powder.
Use a pastry cutter to cut the butter into the flour mixture until a coarse crumb forms.
Add in the egg, milk, and vanilla and mix with a rubber spatula just until combined. The batter will be lumpy. Pour the batter into the prepared baking pan.
In a small bowl, mix together the cinnamon and brown sugar, then sprinkle it over the top of the batter in the pan. Use a butter knife to cut lines through the batter. Sprinkle the top with the chopped walnuts, if desired.
Bake for 25 to 30 minutes, then allow the cake to cool in the pan for 15 minutes on a cooling rack.
Whisk together the water and powdered sugar to make the glaze and drizzle over the top of the cake before slicing and serving.
(Nutrition information provided is an estimate and will vary based on cooking methods and brands of ingredients used.)