Preheat the oven to 350 degrees. Grease a 9-inch square baking dish or cake pan with butter.
In a small bowl, toss the cranberries with ¼ cup of flour; set aside. Whisk together the remaining flour, baking powder and salt in a medium bowl and set aside.
In another small bowl, combine the ingredients for the crumb topping (flour through butter); set aside.
Using an electric hand mixer or stand mixer, cream the butter, lemon zest and ¾ cup plus one tablespoon of the sugar together until light and fluffy. Add the egg and vanilla and beat until combined.
Add the flour mixture to the batter a little at a time, alternating with the buttermilk. Fold in the cranberries.
Spread batter into greased baking dish and sprinkle batter with crumb topping, crumbling with your hands to make large chunks. Bake for 35 to 45 minutes, or until top is slightly golden brown and a toothpick inserted into the center comes out clean. Allow to cool for 10 to 15 minutes.
To make the glaze, whisk the confectioners' sugar, milk and vanilla extract together. Drizzle over cake. Cut into square slices and serve or store in a sealed container for up to 2 days.