Preheat oven to 350°F (175°C). Grease and flour a 9x9-inch (23x23 cm) square baking pan, or line with parchment paper leaving an overhang to lift the cake out later.
Make the crumble: In a small bowl, whisk together the flour, brown sugar, cinnamon, and salt. Pour in the melted butter and mix with a fork until large, moist crumbs form. Set aside.
Mix dry ingredients: In a medium bowl, whisk together the 2 cups of flour, baking soda, ginger, cinnamon, nutmeg, cloves, and salt for the cake.
Mix wet ingredients: In a large bowl using an electric mixer, beat the softened butter with the brown sugar and granulated sugar on medium-high speed until light and fluffy, about 2-3 minutes. Beat in the eggs one at a time, then mix in the molasses, sour cream, and vanilla extract.
Combine: Add half of the dry ingredients to the wet ingredients and mix on low speed just until combined. Pour in the buttermilk and mix until just incorporated. Add the remaining dry ingredients and mix until no streaks of flour remain. Do not overmix.
Assemble and bake: Spread the thick batter evenly into the prepared pan. Sprinkle the crumble topping evenly over the batter.
Bake for 45-55 minutes, or until a wooden skewer inserted into the center comes out clean. An instant-read thermometer should register about 205°F (96°C) in the center of the cake.
Let the cake cool in the pan on a wire rack for at least 20 minutes before glazing and slicing.
Make the glaze (if using): In a small bowl, whisk the powdered sugar, milk, and maple syrup until smooth. Drizzle over the cooled cake.