Wipe down the mixing bowl and whisk attachment with lemon juice or white vinegar to ensure there is no grease on any of your equipment. Set aside.

Prep two baking sheets with parchment paper or a silicone mat.

Heat a small saucepan with 2 inches of water over medium heat until the water simmers. Add the egg whites to a large mixing bowl (the bowl of an electric stand mixer is ideal) and add a few spoonfuls of the granulated sugar. Whisk over the simmering water until the egg whites become white and frothy, about 1 to 2 minutes. The bottom of the bowl should not be touching the water.

Once the egg whites become frothy, transfer the mixing bowl to the stand mixer and add the remaining granulated sugar. Using the whisk attachment, whip on medium-low speed for about 4 minutes. Increase the speed to medium high and whip until stiff peaks form and the mixture is glossy. Add the purple food coloring and continue mixing until evenly incorporated. Remove the whisk and hold it straight up; if the whites hold peaks that stand up without falling, you have whipped enough.

Sift the almond flour and powdered sugar together in a large bowl. Sift three times to ensure there are no lumps. Discard any large pieces of almond flour left in the sifter.

Add the sifted almond flour and powdered sugar mixture to the bowl with the meringue. Using a silicone spatula, gently fold the almond flour mixture into the meringue in circular motions. This will deflate the batter slightly which is necessary when making macarons. Continue this motion until the batter is the consistency of slow-moving lava.

Scoop the batter into a piping bag with a 1-inch round tip and begin piping on the lined baking sheets, leaving 1 inch in between each macaron. After piping the macarons, gently tap the baking sheet on the counter a few times to release any air pockets. Top with Halloween sprinkles.

Let the macarons dry on the baking sheet at room temperature. The amount of time this takes will vary depending on your geographic location and the humidity; the range is anywhere from 30 minutes to 1 hour. Check to see if they're dry by lightly touching the tops of the macarons with your finger; if they're still pretty sticky, dry them out longer. While the macarons are drying, preheat the oven to 300° F.

Once the macarons are dry, bake each pan for 12 minutes at a time. Let them cool fully on the baking sheet before removing them.

While the macarons are cooling, make the chocolate buttercream. Cream the butter with a handheld mixer or stand mixer, until fluffy. Once creamed, add the melted chocolate truffles and vanilla. Mix until smooth. Next, add the cocoa powder and powdered sugar until well incorporated. Add the 5 drops of black food coloring, if desired, to darken the buttercream. Mix until the color is evenly distributed.

Transfer the buttercream to another piping bag with your preferred tip. (I like using the same 1-inch tip for a clean look.)

Pair the macaron shells so the two sides are similar in size.

Pipe the buttercream onto the inside of one of each pair and carefully sandwich the two shells together. Store the macarons in an airtight container in the fridge for up to 5 days.