In a medium mixing bowl, add the softened butter, and granulated sugar. Mix with an electric hand mixer until well combined and creamy.
Add the kosher salt, vanilla extract, milk, and egg, and mix until well combined.
Add the flour and mix until you get a smooth dough. Smooth it out with a spatula.
Transfer the cookie dough to a large piping bag, open a medium size slit, and pipe it evenly as seen in the photos.
Bake in a 180c (350f) preheated oven for 15 minutes or until the edges of the cookies are lightly golden brown. Remove them from the oven and let them cool completely.
Make the chocolate ganache
Melt dark chocolate and heavy cream in a small to medium mixing bowl in the microwave for 30-second pulses or in a bain Marie.
Let the chocolate and the cream sit for 10 minutes for the chocolate to melt, then mix until the chocolate ganache is smooth and silky.
Let the chocolate ganache cool in the fridge until it reaches a spreadable texture.
Fill each cookie with a thin layer of chocolate ganache and top it with another cookie to create a sandwich.
Notes
Use room temperature ingredients. It is important that you use room-temperature ingredients like eggs and butter. This step ensures that your mixture mixes into a homogenous mixture and bakes perfectly. you don’t want to place cold batter into the oven.
Don’t overmix the batter, you don’t want to develop the gluten in the flour. This step ensures you have moist cookies and not dense and dry cookies.
Make sure that your butter is at room temperature. An hour or two before making these cookies, take the butter out of the fridge and let it come to room temperature. The butter should be soft to the touch. If your butter is too cold, it will be harder to mix and there will be lumps of butter in the dough.
Measure your flour correctly!When measuring your flour, avoid scooping it with a measuring cup. Instead, stir the flour with a spoon to create as many air pockets as possible, and spoon it into your measuring cup. Finally, level it with a back of a knife.
Line your sheet pan with parchment paper or a silicone mat! This step helps the cookies to bake evenly, and it makes sure that your cookies release easily from the pan.
Before you start to make this recipe, measure and prepare your ingredients so the cooking process will go smoothly and easily.