In the bowl of an electric mixer, beat the butter until creamy. Gradually add the sugar, being sure to stop the mixer and scrape the sides of the bowl occasionally. Finally, add the cooled coffee and beat until smooth.

In the bowl of an electric stand mixer, beat together butter and sugars until combined and fluffy.

Add in vanilla and eggs and beat until just combined.

Slowly pour in room temperature coffee and beat until incorporated.

Add flour, baking powder, baking soda and 1/2 teaspoon salt. Mix until dough is formed. Cover and chill for 30 minutes.

Preheat oven to 350°F.

Using a small cookie scoop, scoop dough onto Silpat or parchment lined cookie sheets with plenty of space in between each cookie (dough will spread as it cooks).

Bake for 12-14 minutes or until cookies are soft and light brown.

Let cool for 3 minutes, then remove from cookie sheet and cool completely on a wire cooling rack.

Once cooled, pipe frosting onto half of the cookies, then top with an additional cookie, forming a "cookie sandwich". Sprinkle top of cookie sandwich with a pinch of Fleur de Sel or Sea Salt, if desired.

Cookies may be stored in the fridge until ready to serve. Yields approximately 1 dozen cookie sandwiches (or 24 individual cookies).