Preheat oven to 350 degrees. Line a large baking sheet with parchment or silicone baking mat and set aside.
In a large mixing bowl, beat together butter and sugars for about 3-4 minutes.
Combine eggs with espresso powder and vanilla. Mix well and add to butter and sugar. Mix until well combined.
Sift together flour, cocoa powder, baking soda and salt. Add to butter mixture, along with chopped chocolate or chocolate chips and mix until a stiff dough is formed.
Turn dough out onto counter and separate into two equal pieces. Form each piece into a log about 10 inches long by three inches wide. Place onto prepared baking sheet and flatten, leaving at least two inches between the logs.
Bake at 350 degrees for 30 minutes. Remove from oven and set aside to cool for five minutes.
Cut logs into diagonal slices, about ¾” thick. Place back onto baking sheet, cut side down, and bake for an additional 10 minutes. Cool on sheets for 2-3 minutes before removing to cool completely on wire racks. Once completely cool, decorate as desired.
