Acorn meringue cookies are adorable cocoa-flavored meringue kisses dipped in chocolate and coated with ground walnuts. They look like little acorns, but taste like the crispest little chocolatey treat you've ever had!
Preheat oven to 250F and line two baking sheets with parchment paper. Note: it helps to lightly spray or add a few dabs of butter to the tray first so the paper adheres well. This will help you when you are piping out your meringue.
In the (very clean!) bowl of a stand mixer, beat the egg white and cream of tartar at medium/medium-high speed until foamy (a minute or so).
Turn the mixer up to high speed, and with the mixer running, add the sugar about a big spoonful at a time, allowing it to get mixed in for 30 seconds or so between each addition.
Continue whipping the meringue until it is thick and glossy, and add the vanilla extract and cocoa powder.
When the meringue holds stiff peaks, it is done. Be sure to use a silicone spatula to scrape down the sides of the bowl to get everything well incorporated.
Transfer to a piping bag fitted with a large piping tip. I used the Wilton A-1 round tip here. If you use another size, just be aware that your baking time may need to be adjusted for larger or smaller cookies.
To make perfect acorn shapes, hold the tip of the piping bag about 1/2" above the surface of the cookie sheet. Pipe the bottom of the acorn without moving the piping tip, and then lift up quickly at the end for a little pointy tip.
Bake for about 1 hour (this makes meringues that are crisp all the way through). Turn off the oven after 1 hour and crack the door open. Leave the meringues in the oven for another half hour before removing.
When the cookies have fully cooled, melt the chocolate in the microwave or in a double boiler. Place the ground walnuts in a shallow bowl for dipping.
Carefully dip the base of each meringue cookie into the melted chocolate, and then roll it in the ground walnuts to coat.
Place the cookies back onto the parchment lined baking sheet and allow the chocolate to fully harden before moving.
Store them in an airtight container at room temperature. As long as the air is not humid, they will last several days this way.
You can freeze meringue cookies for up to 3 months in an airtight container. Let them thaw on the counter. Do not put meringues in the refrigerator or they will get soft.