Add the fennel seeds to a small pan and place the pan over medium heat. Move the seeds around frequently until they're golden and smell toasty, about 5-6 minutes.
Transfer the toasted fennel seeds to a mortar and pestle. Lightly crush the seeds until fragrant.
In a medium bowl, whisk together the flour, cocoa powder, espresso powder, salt, and fennel seeds.
In the bowl of a stand mixer fitted with a paddle attachment, cream the butter and both sugars on medium speed for 2-3 minutes until light and creamy.
Scrape down the bowl with a silicone spatula then mix in the egg and vanilla extract.
Add the flour mixture and mix on low speed until just combined. Scrape down the bowl once more and mix for a few more seconds on low speed to ensure everything is well combined.
Tip the dough out onto a clean surface and form it into a disk. Cut it in half then form the pieces into two 1 ½-inch (3.8 cm) thick logs. Wrap them with plastic wrap and place them upright in two tall glasses.* Chill in the fridge for at least 2 hours or overnight.
A few minutes before the dough is done chilling, preheat the oven to 350°F (180°C) and line a large baking sheet with parchment paper.
Remove the dough from the fridge and cut the logs into ½-inch (1.25 cm) thick medallions.
Place the cookies on the prepared baking sheet about 1 ½-inches (3.8 cm) apart and bake for 20-22 minutes until the edges of the cookies are set and the tops are matte/no longer glossy.
Cool on the pan for 5 minutes then transfer to a wire rack to cool completely before adding the chocolate.
For the chocolate drizzle, melt the dark chocolate in 20 seconds intervals in the microwave, stirring in between until melted. You can also use a double boiler.
Drizzle the chocolate onto the cooled cookies and allow them to sit until the chocolate is set. You'll know it's set when it's no longer glossy and the tops are dry to the touch. If you want, you can pop them in the fridge for a few minutes to speed up the process.