Preheat your oven to 350º F (180º C). It’s important to have the right temperature for perfect baking. Line two baking sheets with parchment paper, ensuring they’re ready for the macarons.
Using a blender or food processor, grind together the powdered sugar, powdered almonds, and cocoa powder until there are no lumps. This step ensures that your macarons have a smooth texture, so take your time and make sure everything is finely blended.
In a standing electric mixer, beat the egg whites until they begin to rise and hold their shape. Gradually add in the granulated sugar, while continuing to whip until the mixture becomes very stiff and firm, about two minutes. You want to achieve glossy peaks that hold their shape.
Carefully fold the dry ingredients into the beaten egg whites in two batches, using a flexible rubber spatula. Be gentle to avoid deflating the mixture; stop when the batter is just smooth and there are no streaks of egg white. This is a crucial step, so take your time.
Scrape the batter into a pastry bag fitted with a plain tip (about 1/2-inch, 2 cm). Standing the bag in a tall glass can help if you’re working alone. Pipe the batter onto the parchment-lined baking sheets in 1-inch (3 cm) circles, leaving about an inch of space between each.
After piping, rap the baking sheet a few times firmly on the counter to flatten the macarons. This helps to release any air bubbles. Bake the macarons for about 15 to 18 minutes until they are firm to the touch. Once they are done, let them cool completely before removing them from the baking sheet.
While the macarons are cooling, prepare the filling. Cut the prunes into quarters and pour boiling water over them. Cover and let stand until soft, then drain and squeeze out excess water. Pass the prunes through a food mill or food processor until they are pureed.
In a double boiler or microwave, melt the milk chocolate and Armagnac, stirring until smooth. Combine this mixture with the puree. Allow it to cool completely to room temperature; it will thicken as it cools.
In a small saucepan, heat the heavy cream with corn syrup until it just begins to boil at the edges. Remove from heat and add the chopped chocolate. Let it sit for one minute, then stir until smooth. Incorporate the butter pieces and let the mixture cool completely before using.
To assemble the macarons, spread a bit of filling on the inside of each shell, then sandwich them together. You can use a piping bag for this, but spreading by hand is also fun!
Let the assembled macarons stand for at least one day before serving to allow the flavors to meld beautifully. They can be stored in an airtight container for up to five days, or frozen. If you choose to freeze them, defrost in the unopened container to avoid condensation.