Prepare the cherries: Drain the sour cherries in a fine-mesh strainer. Spread them over several layers of paper towels and blot thoroughly. Roughly chop them, then blot again. They should feel moist but not wet enough to leave visible juice on the towel.

Combine the dry ingredients: In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt until the cocoa is evenly dispersed and no large lumps remain.

Cream the butter and sugars: In a large mixing bowl, beat the softened butter, brown sugar, and granulated sugar on medium speed for 2 to 3 minutes. The mixture should look creamy and slightly fluffy rather than oily.

Add the egg and yolk: Beat in the whole egg and egg yolk until fully incorporated. Add the vanilla and mix again, scraping the bottom and sides of the bowl as needed.

Add the dry ingredients: Mix the flour mixture into the butter mixture on low speed just until a thick chocolate dough forms. Stop when only a few small flour streaks remain.

Fold in the cherries and chocolate: Reserve 1/4 cup (40 g) of the chocolate chunks for topping. Fold the remaining 3/4 cup (130 g) chocolate and the dried-off chopped cherries into the dough. Mix gently so the cherries do not release additional juice.

Chill the dough: Cover the bowl and refrigerate the dough for 1 hour. The chilled dough should feel firm enough to scoop while still being pliable.

Prepare the oven and pans: Preheat the oven to 350°F (175°C). Line two large baking sheets with parchment paper.

Portion the cookies: Scoop the dough into 2-tablespoon portions, about 1 1/2 ounces (42 g) each. Roll gently into balls and place them at least 2 inches (5 cm) apart on the prepared baking sheets.

Add the reserved chocolate: Press a few pieces of the reserved chocolate chunks into the top of each dough ball. Do not flatten the cookies.

Bake the first batch: Bake one sheet at a time on the center rack for 10 to 12 minutes. The edges should look set, while the centers should still appear slightly soft and puffed.

Avoid overbaking: Remove the cookies even if the centers do not look completely firm. They will continue cooking from the heat of the baking sheet. Overbaking will reduce their chewy texture.

Cool on the pan: Let the cookies cool on the baking sheet for 10 minutes. They should settle slightly and become firm enough to move without breaking.

Cool completely: Transfer the cookies to wire racks and cool for another 20 minutes. Repeat the baking and cooling process with the remaining dough.

Finish the cookies: Sprinkle with the optional finely chopped dark chocolate while the cookies are still slightly warm, if using. Allow them to cool completely before storing.