In a food processor, add sugar and orange zest. Process until the zest is very fine. If you don’t have a food processor, in a medium bowl, combine sugar and orange zest. Mix lightly.
In a medium bowl, whisk together flour and salt.
In a large mixing bowl with an electric mixer fitted with a paddle attachment, beat the butter until smooth. Add sugar mixture and beat on medium speed until fluffy, about 3 minutes. Scrape down the sides of the bowl. Add egg and vanilla extract. Beat until just blended. With the mixer on low speed, gradually add the flour. Mix until the dough comes together and the flour has disappeared.
Transfer the dough to a large piece of plastic wrap and shape into a ball. Divided into 4 parts and pat into disks. Wrap each disk in a new plastic wrap. Chill in the fridge for at least 2 hours (or up to 2 days).
Preheat oven to 350˚F (177˚C). Keep the oven rack in the center. Line 2 baking sheets with parchment papers.
Once the dough is chilled, prepare a clean working surface and lightly dust with flour. Take out 1 part of the dough and roll into 1/8-inch thick. To prevent sticking, move the dough from time to time and dust with more flour if needed. With a 2¼-inch round scallop cookie cutter, cut out as many cookies as you can. Gently lift the cookie and place them on a single layer side by side on one of the prepared baking sheets. Chill in the fridge. Collect the leftover scraps, shape into a round disk and keep in the fridge to chill again. Repeat with the other 3 parts of the dough. Make sure the cookies have been chilled in the fridge for at least 10 minutes.
Take out 13 cookies and place them on the other prepared baking sheet by spacing ½-inch apart. Bake in the oven until the cookies begin to brown around the edges and the bottom is light golden brown, about 14 – 16 minutes. Transfer the baking sheet to counter and let the cookies rest on the sheets for 2 minutes. Transfer the cookies to a cooling rack to cool completely.
Now, line another baking sheet with a parchment paper. Take out 13 cookies and place them by spacing ½-inch apart. Use a ¾-inch letter or shape cookie cutter, cut out the centers. Bake in the oven until the cookies begin to brown around the edges and the bottom is light golden brown, about 12 – 14 minutes (less than round cookies). Transfer the baking sheet to counter and let the cookies rest on the sheets for 2 minutes. Transfer the cookies to a cooling rack to cool completely.
Repeat with the rest of the dough, about 18 more cookies. Half for round cookies and half for letter or shape cookies. Bake them separately because of different bake time.
For the ganache, in medium heatproof bowl, place chopped chocolate and butter.
In a small saucepan over medium-high heat, bring the cream just to a boil. Remove from heat. Quickly stir in the orange zest. Pour over the chocolate and butter. Whisk until smooth. Transfer to the fridge until spreadable consistency, about 20 minutes, stirring occasionally.
To assemble, spread the ganache onto the flat side of one round cookie, leaving 1/8-inch plain border. Smooth out the top with the back of a spoon or an offset spatula. Top with a second cookie with letter or shape, flat side down. Repeat with the remaining cookies and ganache.