Sift together All-Purpose Flour (3/4 cup), Unsweetened Cocoa Powder (1/2 cup), Baking Powder (1/4 teaspoon), and Salt (1/8 teaspoon). Set aside.
Using a stand mixer fitted with a paddle attachment or electric mixer beat Unsalted Butter (3/4 cup) until fluffy.
Add Granulated Sugar (3/4 cup) and beat until incorporated. Add Egg (1) and Pure Vanilla Extract (1 teaspoon) and continue to beat on medium until egg is incorporated and the mixture is fluffy.
Add dry ingredients to butter mixture and blend together until just incorporated.
Divide dough in two, scooping out onto sheets of plastic wrap. Fold the plastic wrap over to seal and press each into a 1-inch flat disk. Refrigerate for at least 2 hours or overnight.
Heat Heavy Cream (4 fluid ounce) and pour over Semi-Sweet Dark Chocolate (8 ounce) Corn Syrup (3 teaspoon). Allow to sit for 5 minutes.
Whisk together until smooth and glossy. Cover with plastic wrap directly on chocolate. Allow to sit and set at room temperature (see note), this will take several hours.
Preheat oven to 375 degrees F (190 degrees C). Line baking trays with parchment.
Remove dough from fridge and allow to rest at room temperature for about 10 minutes. Dust a work surface with flour and roll dough out to 1/8-inch thickness. Cut into desired shapes and place 1-inch apart on baking trays.
Re-roll remaining dough, dusting with flour as needed. Repeat with the second disk of dough.
Bake cookies for 7 to 8 minutes, allowing to cool slightly on trays before removing to a cooling rack.
Once completely cooled, spread or pipe 2 to 3 tsp of ganache onto the bottom of a cookie. Top with another cookie, bottom side down.
Melt Semi-Sweet Dark Chocolate (2 ounce) with Corn Syrup (1 teaspoon). Pour into a disposable piping bag cut with a very small hole to drizzle chocolate designs over the cookie tops.
Store cookies in an airtight container at room temperature for 2 days, in the fridge for 3 to 4 or in the freezer for up to a month.