Pre-heat oven to 350 degrees.
In the bowl of a stand mixer (or in a large bowl with a hand mixer), cream together butter, brown sugar, and granulated sugar until light and fluffy, about 2-3 minutes.
Mix in egg and vanilla extract until combined.
In a separate medium-sized bowl, whisk together cake flour, baking powder, and salt.
Add half of flour mixture into mixing bowl and blend until just combined.
Mix in coffee creamer and milk.
Add the remainder of the flour mixture to the mixing bowl and blend until just combined.
Lightly spray whoopie pie pan with oil or cooking spray.
Spoon batter into whoopie pie pan cavities, using about 1 ½ T to fill cavities ⅔ of the way full. Make sure batter is spread to edges of pan.
Bake for 7-9 minutes, until cakes are just starting to turn golden around edges and a toothpick comes out clean.
Let cakes cool in pan for at least 5 minutes. Remove cakes from pan and cool completely on baking rack.
Once cakes are cooled, fit pastry bag with a standard round pastry tip and spoon custard filling into bag. Pipe generous amount of custard filling onto one whoopie pie cake round and sandwich with another.
Spoon chocolate ganache over the top of each whoopie pie and allow to drip down the edges.