- Beat the softened butter, sugar, vanilla, espresso powder and salt until smooth using a stand or hand held mixer. Sift the cocoa and flour into the bowl and mix just until a dough forms. Using a spatula stir 1⁄2 cup of the mix-in ingredient into the dough if desired.
- Cut 2 pieces of 12 inch long parchment. Divide the dough in half and place on the parchment pieces. Use the parchment to roll the dough into 8 inch logs. Wrap tightly in the paper and refrigerate until firm, approximately 2 hours or wrap in plastic wrap for up to 3 days.
- Preheat oven to 325 degrees. Slice the logs into generous 1⁄4 inch slices using a serrated knife. Place the slices about 2 inches apart on a parchment lined baking sheet.
- Bake until the edges are set for about 12-15 minutes. Cool on the baking sheet for 5 minutes and then transfer to a wire rack to cool completely.
- Place the coarsely chopped chocolate in a microwave safe bowl and microwave for 30 second increments, stirring in between, until melted.
- Once the cookies have fully cooled, dip a fork into the melted chocolate, drizzle over the cookies or dip the cookies in the chocolate and sprinkle with the topping ingredients if desired.
To Make Ahead:
Store wrapped logs in the refrigerator for up to 3 days. Freeze for up to 3 months. Thaw frozen logs in the refrigerator overnight before baking.