For The Gluten-Free Chocolate Cake Cookies:

1. Preheat the oven to 350 degrees F. Line a baking sheet with parchment paper. 
2. In a stand mixer or large bowl, combine butter and brown sugar. Beat 2-3 minutes, or until light and fluffy, scraping down the edges of the bowl now and then as needed.
3. Add egg and egg yolk and beat to combine.
4. Add vanilla and beat to incorporate.
5. Scrape down the edges of the bowl and add baking soda, salt, cocoa powder, and gluten-free flour.
6. Mix until the flour is just incorporated. The dough will be thick and sticky. 
7. Use a large (3 Tbsp.) cookie scoop to scoop the dough into balls* and place 6 on a baking sheet at a time, spacing the cookie dough balls out evenly. 
8. Gently pat the cookie dough balls into discs with about 1/2″ thick and 2 1/2 inches wide. (I find using just 2-3 fingers moving in circles, helps me pat them out evenly.) This will help them spread properly and set properly when baked. 
9. Bake cookies at 350 degrees 8-9 minutes, or until the centers are just no longer wet/glossy and look just set. The cookies will give slightly when you gently press the centers with your fingers.
10. Allow the cookies to cool on the baking sheet 2-3 minutes before transferring to a cooling rack to cool completely.
11. Repeat with remaining dough until all cookies are baked. Cool cookies completely before frosting them. (Don’t skip this step!)

For The Chocolate Frosting:

1. In a microwave-safe bowl, gently melt the chocolate 10-15 seconds at a time, stirring between each interval, until the chocolate is smooth and glossy. (I like to stop heating it when there are just a few small pieces of chocolate chip left and let the residual heat melt the chocolate the rest of the way. This helps me avoid scorching the chocolate.)
2. Let the melted chocolate cool before you add it to the frosting. 
3. In a stand mixer or mixing bowl, combine softened butter, powdered sugar, cocoa powder, vanilla, and salt. Beat for 2-3 minutes, or until it’s light and fluffy. At first, it will look dry and crumbly. Just keep beating! (See the video for a visual).
4. Scrape down the edges of the bowl and add the cooled melted chocolate (make sure it’s cooled so it doesn’t melt the butter!). 
5. Beat another 1-2 minutes, or until the chocolate is incorporated and the frosting is light and fluffy.
6. Add frosting to a piping bag fitted with a drop flower or star tip (I’m using a Wilton 2D) and pipe the frosting in a spiral on each cookie to form a rosette. Start in the middle of the cookie and work outward in a spiral to make a rosette. Garnish with sprinkles, if desired.

Troubleshoot As Needed. As with all baking, there are a LOT of conditions that can affect your bakes–ambient temperature in the room, the temperature of your butter and melted chocolate, the humidity, etc. If your frosting is too soft to hold its shape, add more powdered sugar 2 Tbsp. at a time until it’s stiff enough to hold its shape. If it’s too stiff, add more liquid 1 tsp. at a time until it loosens up.