A cross between a cake and a soft cookie, these chocolate whoopies are amazing, intense but tender, sweet but not so much depending on the filling. They are very easy to make and the batter can be kept in the fridge for 2 days before baking.
Beat butter with sugar and oil in a bowl for about 1 minute.
Add egg, vanilla, and coffee and mix until well integrated.
Sift the cocoa powder over this mixture and mix.
Add sifted flour, baking soda, and salt in 2 parts. Integrate well but don't overbeat at this point; only mix to incorporate because the cakes will be tough if beaten too much. If using an electric hand mixer mix at low speed.
Cover the bowl and refrigerate for about 2 hours, until firm enough to scoop.
Preheat oven to 350°F (180°C).
Line a cookie sheet with parchment paper.
Scoop portions of chocolate dough and place with an inch or two of separation between them. I use a small or medium cookie scoop at the most as they grow quite a bit during baking.
Bake for 12 to 15 minutes, until crackled, dry, and still slightly soft when barely pressed.
Let cool completely on a wire rack and fill.
Keep at room temperature for a day or two if you want, or put in airtight containers. You can also store the whoopies without filling for a few days and fill when ready to eat.
For the filling:
Mix butter and cheese until creamy and smooth.
Add 1 cup of powdered sugar little by little until it is very thick. Add more sugar if you want a thicker mixture. You might not need to use the whole 1 ½ cups.
Assembly:
Place a teaspoon or two of filling on the flat side of half of the cookies. Spread lightly without reaching the edges.
Top with the second cookie and press lightly to the filling spreads and fills the whole cookie.
Notes
Baking time: keep in mind that all ovens and pans are different, even if they look the same or very similar. The baking time in my recipes is as accurate as it can be, but it might take you more or less time. Use a thermometer inside the oven to check that the temperature is right. I recommend you keep track of how your oven works and what tiny details you might need to adjust. Storing: the soft cookies keep for a few days in an airtight container or jar. You can also freeze them for a month, always well wrapped. Be careful not to smash them. You can also store filled whoopies but they will turn softer faster due to the moisture of the filling. Cocoa powder: the better the cocoa the better the flavor of these soft cookies/cakes. Always use unsweetened, and, in my experience, the dark ones are the best (this does not apply to the super dark used for making homemade oreo cookies known as dark cocoa powder).Fillings: cream cheese frosting and marshmallow creme filling (or marshmallow fluff) are the popular ones, but dulce de leche and peanut butter pair fantastically. If you're a huge fan of chocolate you can use the 5-minute chocolate frosting or the chocolate peanut butter frosting as a filling.