Preheat oven to 350 degrees (F).
Mix together dry ingredients – flour, cocoa, baking soda, and salt in a medium bowl and set aside.
Beat together butter and brown sugar in a large bowl at medium speed until fluffy.
Add egg and vanilla.
Alternately mix in dry ingredients and buttermilk until smooth.
Spoon ¼ cup mounds of batter about two inches apart on a parchment lined baking sheet.
Bake 11 to 13 minutes until top is puffed and springs back when touched.
Cook completely on a wire rack before filling.
Beat together the butter, confectioners' sugar, vanilla and nutella until smooth.
Add in 3 tablespoons of Frangelico liqueur (or milk) and beat filling until smooth and creamy, about 5 minutes on medium speed. If filling appears too stiff, add additional liquid, 1 teaspoon at a time, until desired consistency is reached.
Spread a heaping tablespoon of the filling onto the flat side of one cookie and top with another. Easy Peasy!
If you cook 2 baking sheets at a time, be sure to rotate them front to back and swap the sheets from top to bottom half way through baking time. This will help them bake more evenly.
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