Chocolate Sandwiches with Creamy Filling

1. Preheat oven to 350 F. Combine the cocoa powder and water in a small bowl. Stir until thoroughly combined. Set aside to cool.
2. In the bowl of a stand mixer fitted with a paddle attachment, cream the shortening and sugar. Add the cooled chocolate liquid, eggs and vanilla. Mix to combine.
3. In a separate bowl, combine the flour, baking soda, baking powder and salt. Combine the milk and vinegar in a small measuring cup.
4. Add 1/3 of the dry mixture into the chocolate mixture. Stir to combine. Stream in 1/2 of the milk mixture. Repeat and add another 1/3 of the dry mixture and the remaining milk mixture. Then add the last of the dry mixture and mix until creamy and smooth.
5. Prepare 2 baking sheets by thoroughly greasing them with shortening. Spoon tablespoon size scoops of the chocolate batter onto the pan allowing 2 inches of room between each cookie.
6. Bake at 350 F for 12 minutes.
7. Remove pans from the oven, set them on a rack and allow cookies to cool for a few minutes. Then move the cookies onto a rack and allow them to cool completely.
8. While the cookies are cooling, prepare the filling.
9. In the bowl of a stand mixer fitter with a whisk attachment, cream the butter and cream cheese until fluffy. Add the vanilla, milk and salt. Whisk until combined. Slowly add in the sugar, 1/2 cup at a time. Add enough sugar to make the filling thick enough to hold together but not stiff; you should be able to form soft peaks.
10. Spread the filling onto the bottom of one cooled cookie. Top with another cookie to create a sandwich. Place the assembled sandwich on a tray and repeat with the remaining cookies and filling.
11. Place the sandwich cookies into the refrigerator for at least 20 minutes before serving in order to allow the filling to set.
12. Store extra cookies in an airtight container in the refrigerator.