Preheat oven to 350F and line cookie sheets with parchment paper.
In large, micro-wave safe bowl, combine butter and chocolate chips. Microwave for 30 seconds and stir, repeating at 15 second increments (stirring in-between) until chocolate and butter are completely melted.
Stir in sugars, eggs and vanilla.
In separate, medium-sized bowl, whisk together flour, cocoa powder, cornstarch, baking powder, baking soda, and salt.
Gradually stir flour mixture into butter mixture until completely combined.
Drop cookie dough by rounded 1 ½ Tbsp-sized scoop onto parchment paper, pressing down slightly.
Bake 10-11 minutes, allow to cool on cookie sheet for 5 minutes and then transfer to cookie rack to cool completely.
You'll need a heat-proof bowl that can rest in a medium-sized saucepan without touching the bottom of the pan. I use my KitchenAid bowl, which doesn't touch the bottom of my saucepan but rests on the rim instead.
Fill the bottom of a medium-sized saucepan with about 1 ½" of water.
Set over medium-high heat and bring to a slow simmer as you prepare your frosting.
In a heat-proof bowl, combine sugar, egg whites, ⅓ cup water, cream of tartar, and salt. Stir with a spoon until well-combined, and then use a whisk to beat the mixture until it begins to foam.
Set the bowl over your simmering water and continue to whisk constantly until the sugars have dissolved, about 4 minutes (a good way to check this is to rub a droplet of the mixture between your fingers -- so long as it isn't gritty and you can't feel the sugar you are good to go. You don't want the mixture to get too hot).
Once sugar is dissolved, remove your heatproof bowl and transfer it to your stand mixer.
Beat with whisk attachment for 12-15 minutes -- until stiff peaks are formed.
Stir in vanilla extract.
Transfer frosting to a piping bag (I used an Ateco 846 star tip, but any large tip will work -- you may even just transfer the frosting to a ziploc bag and snip off a corner) and pipe frosting on top of cooled cookies, up to 3" high (I prefer slightly less frosting, but this recipe makes enough to put "high hats" on all cookies).
Place your piped cookies on a baking sheet and place in the refrigerator for at least 10 minutes while you prepare your chocolate coating.
In a medium-sized microwave-safe bowl, combine your dark chocolate chips and canola oil.
Microwave for 30 seconds and then stir well.
Return to the microwave and heat at 15-second intervals, stopping after each one to stir well.
Once chocolate has completely melted, transfer to a smaller bowl (I chose a tall, not very wide bowl, something ideal for dipping the cookies so that the frosting does not hit the bottom of the vessel).
Allow to cool for 10-20 minutes (if your chocolate is particularly warm, you may need to wait longer. The chocolate should NOT feel warm to the touch, or it will melt your frosting).
Once chocolate has cooled, remove cookies from the refrigerator.
Holding each one firmly by the base, turn each cookie upside down and quickly dip the frosting entirely in the chocolate and return to right-side-up.
Return to baking sheet and decorate with sprinkles, if using.
Repeat until all cookies have been dipped, and return to refrigerator for at least 10 minutes for chocolate to harden.