Preheat oven to 350 degrees Fahrenheit. Grease a 9-inch square or round baking pan.
Make crumb topping: whisk flour, brown sugar, and cinnamon. Cut in cold butter until clumpy crumbs form. Refrigerate until needed.
Whisk all dry cake ingredients together in a large bowl.
Whisk all wet cake ingredients together in a separate bowl until smooth.
Pour wet into dry and stir until just combined. Batter will be very thick.
Spread batter evenly into prepared pan. Pour crumb topping over the top and gently press it into the surface.
Bake 30 to 35 minutes until a toothpick inserted in the center comes out clean or with a few moist crumbs.
Whisk icing ingredients together until smooth. Drizzle over warm cake. Serve warm.