Preheat oven to 400 °F. Whisk the flour, xanthan if using, baking powder, salt, spices, and sugar together in a large bowl. Be sure to break up any clumps of brown sugar. Add the oil, buttermilk. eggs and vanilla extract and whisk until smooth.
Spoon the batter into a paper-lined muffin tin, filling each about ⅔ full. You should have 1 dozen cupcakes. Bake for about 15-18 minutes, or until a toothpick inserted in the middle of the cupcake comes out clean. Cool completely on a rack.
Prepare the cream cheese frosting. Place the cream cheese and butter in the bowl of your mixer and beat to combine. Add the sugar, vanilla and salt and beat until smooth. Remove from mixer, place in one side of a large zip-top bag (fitted with a large star piping tip if desired) and set aside.
Clean the mixer bowl, then add the cream cheese and pumpkin and beat until combined. Add the brown sugar, pumpkin pie spice and vanilla extract and beat until smooth. Add a drop or two of orange food coloring, if desired and mix to combine. Spoon the frosting into the other side of the large zip top bag next to the plain cream cheese frosting.
If you haven't already, snip off a corner of the bag and carefully twist the top of the bag to force the frosting down in the bag.
Pipe (or swirl) the frosting decoratively over each cupcake. Chill until ready to serve.