1 cup (100 grams) powdered sugar
½ cup almond flour (50 grams)
3 tablespoons (25 grams) unsweetened Dutch-process cocoa powder
2 large egg whites, at room temperature
5 tablespoons (65 grams) granulated sugar
½ cup heavy cream
2 teaspoons light corn syrup (optional)
4 ounces (120 grams) bittersweet or semisweet chocolate, finely chopped
1 tablespoon (15 grams) butter, cut into small pieces
Preheat your oven to 350° Fahrenheit. Stack two cookie sheets together, and line the top one with parchment paper (this isn't necessary, but in my experience gives you the best results). Prep three stacks of cookie sheets — if you don't have enough, just lay out three sheets of parchment paper.
Sift or grind together the powdered sugar, almond flour, and cocoa powder. Set aside.
In the bowl of a standing electric mixer, beat the egg whites, gradually and then increasing the speed to high, until they begin to rise and hold their shape. While whipping, sprinkle in the granulated sugar, and continue beating a few more minutes until you form stiff peaks.
Carefully fold the dry ingredients into the beaten egg whites with a flexible rubber spatula. Continue to mix until it becomes a batter that will pour off of your spatula in a thick-yet-pourable (not plop-able) stream — the party line in macaron-making is going for something that "flows like magma". You want it so that when the batter pours down it will hold its shape for a few seconds, but then gradually slump down into the remaining batter. Yes, you will be deflating things. But that's okay. Think about the fact you're going to pipe cookies that you want to hold their shape somewhat, but not maintain the peak from where you piped them.
When the mixture has reached this stage, transfer it into a pastry bag, or plastic bag (if the latter, then snip off a corner). Pipe one-inch circles onto your parchment paper, with about an inch between them (I just aim for as small as I possibly can). Repeat with remaining batter and sheets.
When the cookies are all piped, rap the baking sheet against the counter once or twice to pop air bubbles (if you have free range sheets of cookies, you can lift up the parchment by both sides, and then drop it from a height of a few inches). Bake about 12-15 minutes, until the cookies are set just enough on the bottom where you can almost peel one off. Remove, let set on the sheet a few minutes, and then remove the cookies to a cooling rack. Repeat with remaining cookies — if you're recycling cookie pans, let them cool slightly between batches. And don't worry about the cookies sitting out on the counter awaiting their turn in the oven — some recipes actually recommend that.
When the cookies are baked and cooled, heat the cream and corn syrup in a small saucepan. When the cream just begins to boil at the edges, remove from heat, and add the chopped chocolate. Let sit one minute, then stir until smooth. Stir in the pieces of butter. Let cool until it becomes thick-yet-still spreadable (I kept trying to speed this up in the fridge, then missing my window and needing to microwave it — seriously like 3 times). When the ganache is ready, take a spoon or knife or small spatula, and place a small spoonful of ganache on the flat bottom of a cookie. Find a similarly sized cookie (if yours, like mine, um, vary a bit), and sandwich them together. Let age a day before eating, or store in the freezer for a few weeks.