Pre-heat the oven to 350 degrees.

In the mixing bowl of a stand mixer, blend the softened butter, powdered sugar, salt, and egg, until creamy. Add the flour a little at a time until well blended, and thick. Scrape down the sides of the mixing bowl and mix again.

Roll the cookie dough into 20 - 24 balls, and place on a parchment paper lined rimmed baking sheet. Bake for 10-12 minutes, or until golden brown.

Before removing from the oven, press down the center of the cookies with a tablespoon, gently, making a small indentation in the center of the cookie. Remove from oven to a wire rack to cool completely.

While the cookies are cooling, make the pastry cream filling. Using a heavy bottom sauce pan, combine the milk and ¼ cup of sugar, bring to a boil, and remove from heat.

In a medium size bowl, whisk the egg yolks, ¼ cup of sugar, and vanilla, until it's a light yellow color, and smooth. Add the cornstarch and whisk again until smooth.

Add ¼ cup of the warm milk mixture, and mix into the egg mixture, tempering the eggs so they don't cook and scramble. Whisk until completely blended, and then add another ¼ cup of the warm milk mixture to the egg mixture, and whisk until smooth again.

Slowly pour the egg mixture into the sauce pan where the milk mixture is, and, while continuously whisking add until all the egg mixture is blended with the milk mixture.

When cookies, are completely cool, and Pastry Cream is chilled, make the Chocolate Ganache.

Place the Chocolate and the heavy cream in the microwave, in a medium size bowl. Microwave on high at 10 second intervals, stirring in-between, until the chocolate is completely melted, and blended with the cream. Remove from microwave.

Fill the center of each cookie with the chilled pastry cream.

Pipe the Chocolate Ganache over the top of each cookie. Use an offset spatula to flatten the ganache gently over the custard. Allow the Chocolate to set, or harden slightly, about 30 minutes inside your fridge.

Serve.