Step 1

Preheat oven to 350°F. Line two rimmed 18-by-13-inch baking sheet with parchment paper.

Step 2

In a stand mixer fitted with a paddle attachment, beat the sugar and butter on low speed until blended. Increase the speed to medium and beat until the mixture is pale yellow and fluffy, scraping down the bowl once or twice, 2 to 5 minutes. Beat in the egg. Scrape down the bowl once more. In a small bowl, mix the buttermilk and vanilla; on low speed, beat into the butter mixture.

Step 3

In a separate medium bowl, stir together the all-purpose flour, rye flour, baking soda, and salt. Add the flour mixture to the butter mixture in the stand mixer. Beat on low speed until the dough comes together (the dough will seem sticky but you want it to look that way!).

Step 4

Using a 2-inch cookie scoop, scoop out 8 cookies and arrange them on the sheet pan, spaced at least 1 1/2 inches apart (alternatively, you can use a 1 1/2-inch cookie scoop to make 12 cookies; reduce baking time accordingly). Bake until the edges of the cookies are golden brown, 12 to 15 minutes. Transfer to a wire rack to cool completely, about 2 hours and up to overnight.

Step 5

Place dark chocolate and corn syrup in a medium bowl; place ruby chocolate and salt in a separate medium bowl. Heat 1 cup heavy cream in a small saucepan until just about to simmer. Pour 1/2 cup hot cream into each bowl of chocolate; let stand for 1 minute. Using separate spatulas or whisks, stir the ruby chocolate ganache until chocolate is melted; repeat with dark chocolate ganache. Set aside both bowls to cool slightly until thickened enough to spread on the cookies, about 10 minutes. Transfer each ganache to a piping bag and snip a small tip off the end of each.

Step 6

To decorate, turn all the cookies so their flat bottoms are facing up on the wire rack. Pipe dark chocolate ganache over half of each cookie to form a semi-circle; smooth the chocolate with an offset spatula. Pipe ruby chocolate ganache on the other side, taking care to create a sharp, clean line in the middle; smooth with a clean offset spatula. Let the chocolate set, about 20 minutes.

Step 7

If using the gold leaf, use a small paintbrush to dab small pieces of gold leaf on top of the cookies. Enjoy!