Preheat the oven to 350°F/180°C (160°C for a fan oven), and line a baking sheet with parchment paper or a Silpat silicone baking mat. Set aside.

In a medium bowl, combine flour with cocoa powder. Set aside.

In a separate mixing bowl, with a hand mixer, cream the butter and sugar until light and fluffy. Add egg and vanilla extract, and beat again.

Add melted chocolate, and beat again until just combined, then add the dry ingredients and beat on low speed until just combined.

Allow the cookies to cool on the pan for 2 minutes, then transfer to a wire rack to cool completely.

In a large bowl with a hand mixer, or in the bowl of your stand mixer, beat the butter at medium speed for 1 minute, or until creamy.

Decrease the speed to low, and gradually add in the powdered sugar, vanilla extract, and salt. Continue beating for 3 minutes until light and fluffy.

If needed, add a small amount of milk to the frosting to get it to the perfect piping consistency.

Divide the frosting in half into two separate bowls. Use food coloring to tint one bowl yellow, and the other with orange color.

To combine the frostings, lay out two pieces of plastic wrap. Add the frosting to the wrap, and then roll it up into a tube shape, twisting the ends to seal. Snip off one of the twisted ends from each tube.

Add a star tip to a large piping bag, then add both colors of frosting, with the open end of the rolls pointing down toward the tip.

Add a swirl of buttercream to each cookie.

Carefully melt the chocolate chips in the microwave at 15-second increments mixing in between. Add coconut oil if needed to thin out the consistency.

Line a cookie sheet with parchment paper.