Cherry Filling:
In a saucepan, combine cherries, sugar, and lemon juice. Cook over medium-high heat, stirring occasionally, until cherries soften and release juices.
Spoon out some juices and mix with cornstarch to make a slurry, ensuring there are no lumps.
Return the slurry to the saucepan, stirring frequently. Bring to a boil and cook for 1 minute.
Test the thickness of the filling by placing some on a plate in the fridge. If the consistency is satisfactory, allow it to cool completely in a cold water bath. For thicker filling, sieve the cherries, cook the liquid further, then recombine and chill.
Flourless Brownie Cookies:
Preheat the oven to 350°F (180°C) and line two baking sheets with parchment paper.
Melt the chocolate and butter together until smooth and glossy, then set aside to cool slightly.
In a mixer, whisk sugar and eggs until pale, fluffy, and tripled in volume.
Pour the chocolate mixture into the whisked eggs and mix until combined.
Sift in cocoa powder and salt, and whisk until smooth. Chill the batter in the fridge for 6-8 minutes.
Scoop 16 cookies using a 2 tablespoon scoop onto the lined sheets. Bake for 8-9 minutes until glossy and cracked. Allow cookies to cool on the sheet for 10 minutes before transferring to a wire rack.
Vanilla Whipped Cream:
Whip the cream, powdered sugar, and vanilla to soft peaks. Place in a piping bag with a star nozzle.
Assembling the Black Forest Cookies:
Turn one cooled cookie upside down and pipe a layer of whipped cream.
Spoon cherry filling over the cream, then top with another cookie.
Optional: Serve without the top cookie for a balanced flavor profile, or include it for a more chocolatey experience. Serve with a fork.
Storage:
Keep assembled cookies in an airtight container in the fridge for 3-4 days.