In a large microwave safe bowl, melt ¼ cup of butter. Mix in maple syrup and vanilla. Add flour, cocoa, baking soda and salt and mix until well combined. Shape mixture into a 1.5 inch diameter log (about 10 inches in length) and wrap with a silicone baking sheet, wax paper or plastic wrap. Chill in the refrigerator for 1 hour and up to 3 days.
Preheat the oven to 300 °F. Line 2 cookie sheets with a silicone baking sheet or parchment paper. Unwrap chilled dough and set on a cutting board. Using a sharp knife, cut the log into thin rounds (about ¼-1/8 inch- you will get around 48-60) and place onto the prepared baking sheets.
Bake for 15 minutes then allow to cool completely on the cookie sheet.
Once the cookies are completely cool, melt the remaining ¼ cup of butter and mix in the icing sugar until smooth. Spoon or pipe the filling onto half of the cookies, then place the other cookies on top of each frosted half to make sandwiches. Refrigerate for 10 minutes to allow filling to set.
Notes
LeftoversStore leftover cookies in an airtight container at room temperature for up to 4 days, refrigerate for up to 1 week or freeze for up to 3 months.