Combine butter, sugar, corn syrup, baking soda, salt, and coconut extract (if using), in the bowl of a stand mixer fitted with a paddle attachment.
Mix on low speed to moisten, then increase the speed to medium and beat until fluffy and light (~5 minutes).
You can use a hand mixer if you don’t have a stand mixer.

While the stand mixer runs on low, add the flour/cocoa mixture and continue mixing until a stiff dough forms.

Gather the dough, press into a ball, and divide it in half. Flatten each half into a disc. The stove may be used immediately, or wrapped in plastic and refrigerated for up to 1 week.

Line cookie sheets with parchment paper.
Preheat your oven to 350°F (177°C).

Generously sprinkle your work surface with cocoa powder, then roll a portion of the dough until it is 1/8 inch (4 mm) thick.

Cut the desired shapes, and transfer them to the parchment paper-lined cookie sheet. You may need to use an offset spatula to loosen them from your work surface.
Space them about 1/4 inch (6 mm) apart. The cookies will not spread while baking.
Gather the dough scraps, knead, re-roll, and cut again.

Bake the cookies until they are firm and dry. Start checking for doneness after about 11 minutes. Depending on your oven, it may take 13 or 14 minutes total.
Since the dough is very dark colored, you want to err on the side of the cookies being very slightly underbaked. It will be very difficult to tell whether they have burned if you overshoot the cooking time

This filling cannot be prepared in advance. It will need to be used immediately after preparing it!

In a large pot, melt the butter over medium-low heat, then simmer, stirring constantly with a heat resistant spatula.

The butter will eventually start hissing and popping. If you notice brown bits forming along the edges, reduce the heat or remove the pot from the heat temporarily.
Continue cooking and stirring until the butter falls silent.
It’s important that you cook off most of the water in the butter. Keep the heat relatively low, and proceed slowly. This step requires patience.

Transfer the butter to the bowl of a stand mixer, fitted with a paddle attachment. Add the vanilla and salt, followed by the sifted powdered sugar.

Mix on low speed to moisten, then increase to medium. Beat until creamy and soft, about 5 minutes.

Turn half of the wafers upside down in order to prep them for filling.
Each of the cookies will be filled with about a tablespoon of filling.

The cookies can be stored in an airtight container at room temperature for up to one week, or up to a month in the fridge.

Once the filling has been added to the bottom wafers, top with the remaining wafers, and join them together with a gentle twist.