Using both light and dark brown sugar in these Autumnal apple butter whoopie pies make them caramel and rich molasses flavored, while the cakes have a light crumb texture and the cream filling has a silky finish you only get with sugar, egg and emulsified butter.
¼ pound (1 stick) Unsalted Butter, at room temperature
1 cup Light Brown Sugar
1 teaspoon Vanilla
1/2 teaspoons Baking Soda
1/2 teaspoon Baking Powder
1/2 teaspoon Kosher Sea Salt
2 cups All-Purpose Flour
1/2 cup Apple Butter, homemade or store bought
1/2 cup Sour Cream
3 Egg Whites
¾ cup Dark Brown Sugar
½ pound Salted Butter (2 sticks - 16 Tablespoons), at room temperature
Preheat the oven to 350.
FOR THE CAKES:
In a mixing bowl, cream together the butter and light brown sugar. Add the egg and vanilla, beat until light and creamy.
In a separate bowl, whisk together the baking soda, baking powder, salt and flour.
In another separate bowl, whisk together apple butter and sour cream.
Add dry ingredients to butter mixture, alternating with the apple butter mixture, until you have a smooth batter.
Using a piping bag, or with a spoon, portion out 50-52, 1.5" rounds of batter allowing ample space for some spread of the batter onto wax paper lined baking sheets. I bake two sheets of 13 at a time.
Bake for 12 to 14 minutes, cakes will be done when a toothpick comes out clean if inserted into a cake.
Remove from oven and cool completely before filling.
FOR THE CREAM FILLING:
Fill a medium saucepan with 2-3 inches of water, and bring to a boil over high heat. Once boiling, reduce to a simmer.
Combine egg whites (reserving yolks for another use) and dark brown sugar in the metal mixing bowl of a stand mixer (or if not using a stand mixer another metal bowl) and place over the simmering water.
Whisk while heating until the sugar is dissolved and mixture temperature reaches 180 degrees.
Using a whisk attachment on a stand mixer (or hand mixer) whisk egg whites and sugar on high until they double in volume. Mixture will be thick and shiny.
Reduce speed to medium and begin to add butter about 1/2 tablespoon at a time, until all the butter is incorporated. It is important that butter be room temperature and that you allow it to fulling incorporated between each mix in.
NOTE: If mixture looks broken, increase speed to high and whisk until smooth. If the mixture looks too lumpy, the butter is too cold, applying heat to the bowl while mixing will help smooth it out. You can apply heat with a torch or holding a hot pot against the side of the bowl while it mixes. If the mixture looks too runny, then the butter is melted. You can remedy that by holding an ice pack against the bowl while it mixes. You will see the frosting tighten up. This may seem fussy, but the texture of this cream filling is worth it!
Fill piping bag with cream filling (or using a spoon), top half of the baked cakes with filling.
Pair up halves and store cookies at room temperature for 3 days or a week in the refrigerator. Cakes can also be store frozen. As per the high butter content, if stored in the refrigerator, the cream will become stiff, so to enjoy soft, you will need to let the pies come to room temperature once removed from the refrigerator.
If desired, dust assembled cakes with a mixture of powdered sugar, cinnamon and clove to add a little extra spice.
Makes 24-27 sandwiched cookies.