Preheat the oven to 350 degrees F. Grease an 8"x8" square baking pan and set aside.
In a medium mixing bowl, whisk together all-purpose flour (1 ¼ cups), baking soda (½ teaspoon), baking powder (½ teaspoon), and salt (⅛ teaspoon). Set aside.
In a large mixing bowl, use a stand mixer to cream unsalted butter (6 tablespoons, room temperature) for about 30 seconds. Gradually add granulated sugar (¾ cup) while continuing to cream until the mixture is fluffy and light.
Add eggs (2 large), one at a time, beating well after each addition.
Add vanilla extract (½ teaspoon) and mix well.
Alternately add the dry ingredients and sour cream (¼ cup) to the butter mixture, beginning and ending with the flour mixture. Mix until just combined; the batter will be thick.
Spread the batter evenly into the prepared pan.
To prepare the topping, in a medium mixing bowl, stir together light brown sugar (¼ cup), granulated sugar (¼ cup), molasses (1 tablespoon), cinnamon (1 teaspoon), and salt (⅛ teaspoon). Add melted unsalted butter (6 tablespoons, slightly cooled) and stir until it forms a thick dough.
Breaking the dough into pieces with your fingers, sprinkle the crumb topping evenly over the batter in the pan.
Bake the cake in the preheated oven for 30 to 35 minutes, or until a toothpick inserted in the center comes out clean.
Once baked, cool the cake in the pan to allow the flavors to settle.
