Preheat your oven to 325 degrees F and grease a mini whoopie pie pan.
In a large bowl, place the flour, xanthan gum, baking powder, baking soda and salt and whisk to combine well.
Whisk in the brown sugar, making sure you remove any lumps.
Next, mix in the butter, shortening, egg and vanilla and make sure everything is well combined.
Now, add the milk and vinegar, and mix again to combine well. You will have a batter that is somewhere between cake batter and cookie dough in consistency.
Using a small scoop or you can pipe about as much as a large marble to each well of your whoopie pie pan.
Bake on a center rack for 8-10 minutes, or until the cakes bounce back when pressed gently in the center. Remove from the oven and allow to cool in the pan for 10 minutes before transferring to a wire rack to cool completely. Repeat with the remaining batter.
Make the pastry cream
Prepare an ice bath.
In a small mixing bowl, combine ¼ cup sugar, egg yolks and the creamer (or ½ cup milk).
In a medium sauce pan over medium heat, bring the remaining 1½ cups milk to a boil, stirring constantly.
Remove from heat and add the hot milk to the egg mixture about a third at a time to temper the eggs. If you do this too fast you will end up with scrambled egg yolks and not a smooth cream.
Return the pot to the stove and add the egg mixture. Over medium heat, keep whisking until the cream thickens and when you scrape the bottom of a pot, the cream does not close the gap immediately. Stir in the vanilla, salt and butter.
Put the pot over your ice bath and stir every 5-10 mintues for about half an hour, or until the cream has cooled.
