Preheat oven to 350.
Make Ganache: Place chopped chocolate into a heat proof bowl.Bring the cream to a low boil. Pour over chocolate. Let sit for 2 minutes then lightly whisk to create a smooth sauce. It will be thin at this stage. But will cool down and thicken to spread on cookies. Cookies: Cream together butter and sugar
Add the egg white and combine. While mixer is mixing, add the whole egg a little at a time. Fully incorporate each portion before adding another. Add in vanilla and milk. Add the flour and gently mix to combine. Put batter into a piping bag.
Pipe the batter on baking sheets in 3 inch long and 1/5 inches apart. Bake for 15 minutes, rotating the pans half way through bake. Until golden brown on the edges. Transfer to cooling rack. Fill cookies when they are fully cooled. Pipe or spoon ganache on flat side of cookie and sandwich with another half. Refrigerate to help ganache set.