For the chocolate cookies:

1. In the food processor: put dry ingredients in the processor's bowl. Mix to combine. 
2. Add butter cubes and pulse a few times to mix. The butter should be the size of peas, roughly. 
3. Add the egg yolk and vanilla if using, and pulse a few times until it moistens the mixture but be careful not to start making a ball. 
4. By hand: put dry ingredients in a large bowl. Stir to combine. 
5. Scatter butter cubes on top. Use your hands or a pastry cutter to incorporate the butter into the flour until it's the size of peas. 
6. Add egg yolk and vanilla if using, and mix with a fork until moistened. The dough will be irregular, as it's harder to incorporate it evenly by hand. 
7. For both methods: dump the shaggy mass onto a counter or working surface.
8. Start bringing it together with your hands. At first, it will appear never to bind or come together correctly. But it will. 
9. Keep turning it onto itself until you have a solid dough. 
10. Wrap it in a plastic or freezer bag and refrigerate for at least 1 hour. This is a crucial step for getting tender and flaky dough that's easily rolled. 

For the frosting:

1. In a medium bowl, beat cream cheese and butter until smooth and creamy. 
2. Add powdered sugar in parts and beat until you have a very thick filling that is lump-free. You might need more or less of sugar depending on the butter and cream cheese you use. 
3. Add extract, beat until smooth and keep refrigerated covered, until ready to use. 

Assembling the sandwich cookies:

1. Take the chocolate dough from the refrigerator until you can roll it. It has to be cold, and it will be tough at first to roll. 
2. Preheat the oven to 350°F/180°C.
3. Line a baking sheet with parchment paper or butter the pan. 
4. Lightly flour a clean surface and roll the dough about 1/4 inch thick. You want thin cookies as they will be filled. 
5. Cut cookies with your designated cookie cutter and place them on the baking sheets, separated an inch from each other. 
6. Bake for 10-12 minutes, until dry and slightly soft. They will harden when they cool. 
7. Let cool for a few minutes and carefully lift them with a spatula, and transfer to a wire rack. Let cool completely.
8. Fill one cookie with 1/2 to 1 teaspoon of cream cheese filling and press another cookie on top to make a sandwich. 
9. Keep in airtight containers. See Notes below on how to store them.