Preheat the oven to 350 degrees F. Rub some butter along the inside of a 9 inch springform pan.
CAKE
In a medium bowl, whisk together the flour, baking powder, baking soda, salt and spices. Set aside.
In a large bowl, use an electric mixer to combine the butter and sugar until light and fluffy. Beat in the sour cream, eggs, molasses, and vanilla. Add the dry ingredients, and mix just until combined. Scrape the batter into the prepared pan.
TOPPING & FINISHING
In a medium bowl, combine the flour, brown sugar, allspice and salt. Add the butter and cut it into the dry ingredients using a pastry cutter, two knives or a fork until crumbly.
Sprinkle the topping on top of the batter in the pan.
Bake until golden brown, about 50 minutes. Let cool at least 20 minutes, then cut into slices and serve.