CUPCAKES: Preheat the oven to 350 degrees and line a muffin tin with paper liners. In a large bowl, cream butter and brown sugar until light and fluffy. Beat in egg and mix until combined. Beat in buttermilk and molasses, mixing until combined. In another large bowl, combine the flour, cinnamon, baking powder, baking soda, salt, ginger, nutmeg and allspice. Whisk until combined. Add dry ingredients to the wet ingredients. Beat on low speed until completely combined and smooth. Beat on medium speed for another 1 minute.
Fill muffin tin cavities two-thirds the way full. Bake for 20-25 minutes or until a toothpick inserted in the center comes out clean. Cool for 10 minutes before removing to a wire rack to cool completely. Cool COMPLETELY before adding frosting (see next step).
FROSTING: in a large bowl, beat cream cheese and butter until completely smooth and creamy. Add vanilla, cinnamon, and salt beat until combined. Add the powdered sugar in 1 cup additions, beating in between each addition until smooth. Add 3 cups for a stronger cream cheese flavor (less stiff frosting) and 4 cups for a sweeter, stiffer (less cream cheese flavored) frosting. Frost cupcakes with a butter knife or by transferring the frosting to a piping bag fitted with a tip. Store cupcakes in the fridge.