Preheat your oven to 350°F (175°C) and line two large baking sheets with parchment paper.
In a large bowl, cream together the softened butter and powdered sugar until light and fluffy (about 3 minutes).
Slowly add the egg whites, vanilla extract, and almond extract, mixing until fully incorporated.
Sift in the flour and salt, then gently fold with a spatula until just combined (do not overmix).
Transfer the batter to a piping bag and pipe 2-inch long strips onto the parchment paper, leaving 2 inches of space between them.
Bake for 8-10 minutes until the edges are just starting to turn light golden brown.
Let the cookies cool completely on the baking sheet; they will crisp up as they cool.
For the filling: Melt the dark chocolate and heavy cream together in 20-second bursts in the microwave until smooth.
Add the peppermint extract to the ganache if you are making mint milano cookies.
Once the ganache has cooled and thickened slightly, spread a teaspoon onto the flat side of one cookie and sandwich it with another.
Let the cookies set for about 30 minutes before serving to ensure the chocolate is firm.