Melt 4 tablespoons of the butter in a medium saucepan. Stir in the cocoa powder and espresso powder to make a smooth paste. Let cool for 15 minutes.

In a standing mixer with the flat beater, cream together the remaining butter with the sugar and the butter/cocoa mixture. Mix on high for 3-4 minutes, or until it is light and fluffy.

Add the egg yolks and vanilla. Mix on medium until combined. Scrape down the bowl and mix for 1 minute.

Stir together the flour and salt.

Set the mixer to low. Add the flour by thirds (⅓ at a time) and mix each addition until just combined, scraping down the bowl of the mixer before adding the next.

Once you've added all the flour, take the bowl off the mixer and use a spatula to finish mixing and make sure you have a cohesive dough.

Transfer the dough to a piece of parchment paper and then shape the dough into a rough log, about 12 inches long.

Wrap the parchment paper around the log so that it wraps around the log and covers it completely with some room on both ends.

Take the top side of the parchment and tuck it in against the log to make a fold. Use a bench scraper to press along the fold and even out the log of the dough into a smooth cylinder.

Chill the dough until completely firm, at least 1 hour and up to 3 days. (If you are storing the dough in the refrigerator for more than a few hours, put it in a sealed bag.)

Heat oven to 375F. Line baking sheets with parchment paper.

Slice the cookies ¼ inch thick. Bake for 10-12 minutes. Then rest the cookies on the tray for 5 minutes before moving them to a cooling rack.