In a large bowl, cream together the butter, eggs, brown sugar and heavy cream. Add the baking soda, cinnamon, vanilla and salt. Mix well. Add in the flour and mix until you have a soft dough. Chill the dough for 1 hour.
Roll the dough out 1/4" thick and use a 3" round cookie cutter to cut circles. You should get 48 circles, to make 24 sandwiches if they are this size.
Preheat the oven to 350 degrees Fahrenheit.
Bake the cookies on a parchment paper lined baking sheet for 8 minutes, or until barely browned.
Make the frosting:
Beat 1 cup (2 sticks) butter, softened, in large bowl of an electric mixer on medium speed until light, fluffy and almost white in appearance.
Add 1 teaspoon vanilla; mix until blended.
Add in 1 tablespoon cinnamon and 2 cups powdered sugar. Beat until well blended.
Add 3 tablespoons of heavy cream and beat until light and fluffy, about 3 minutes. Add more powdered sugar to stiffen the buttercream, or more heavy cream to make it more spreadable, as you desire.
Using a knife, spread the buttercream on the bottom of one of the cookies. Place another cookie on top of the buttercream to make a sandwich. Repeat with the remaining cookies.
Store in an airtight container in the refrigerator.
The calories shown are based on the recipe making 24 cookie sandwiches, with 1 serving being 1 sandwich. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate.