Preheat oven to 350 degrees and line a baking sheet with parchment paper. Draw 2 ½-3 inch lines on the parchment paper, a couple of inches apart. This will be for later when we are piping the dough.
Beat the butter and powdered sugar together on medium speed until light and creamy. Add the egg whites and vanilla extract and mix on medium speed until well combined.
Add the flour and salt and mix on low speed until a creamy dough forms. It will be sticky and impossible to roll, which is why we use a piping bag.
Put the dough in a piping bag with a ½ inch round tip and pipe the dough on the previously drawn out lines. These cookies will spread a lot, so make sure there is enough room in between cookies. Bake the cookies for 9-11 minutes until crisp on the edges.
While the cookies are baking, make the filling. Chop your chocolate and place it in a heat-proof bowl. Warm the heavy cream in a sauce pan over medium heat until it begins to simmer (not boil). Pour the warmed heavy cream over the chocolate and let it sit for 3 minutes. The finer your chocolate is chopped, the quicker it will melt.
After the three minutes, use a rubber spatula to gently fold the chocolate together until combined. Once the cookies are done and slightly cooled, place a thin layer of chocolate on one cookie, and press another on top of the chocolate. Repeat with all the cookies. Let the chocolate harden at room temperature, or in the refrigerator/freezer. Then enjoy!