Whisk together the streusel's brown sugar, flour, ginger, cinnamon, and salt. Add cooled melted butter and mix with a fork until thick crumbs form. Refrigerate while preparing the batter.
Preheat oven to 350°F. Grease a 9×9-inch baking pan.
In a medium bowl, whisk together the cake flour, baking powder, salt, cinnamon, ginger, cloves, and nutmeg. Set aside.
Using a hand mixer, cream softened butter and light brown sugar on medium speed for 2–3 minutes until lighter and fluffy.
Add eggs one at a time, mixing after each addition. Mix in vanilla extract and molasses.
On low speed, alternate adding dry ingredients and buttermilk, beginning and ending with dry ingredients. Mix just until combined.
Spread the thick batter evenly in the prepared pan.
Remove streusel from refrigerator. Break into small and medium crumbs, then sprinkle gently and evenly over the batter without pressing down.
Bake at 350°F for 50–55 minutes, or until a toothpick inserted into the center of the cake comes out clean or with a few moist crumbs.
Cool in the pan for at least 5–10 minutes before serving. Cool longer for cleaner slices.