Preheat oven to 350°F and spray a 9x13 baking dish with cooking spray.
Bring lambic to a boil in a small saucepan. Reduce heat to medium and cook about 10 minutes more, until reduced by about ⅓. Set aside.
Combine chocolate and butter in a microwave-safe bowl. Heat at 30 second intervals, stirring after each one, until melted. (You can use a double-boiler for this if you want, but I'm lazy and I haven't burned chocolate in the microwave yet!)
Combine sugar, eggs, and vanilla in a large bowl and whisk until well-combined. Slowly whisk in chocolate butter mixture, then lambic. Stir in flour and salt, then gently fold in cherries.
Pour brownie batter into baking dish. Bake for 30-35 minutes or until toothpick inserted into center of dish comes out clean. Cool on wire rack for at least 20 minutes and cut into 16 squares. Serve with vanilla ice cream and cherry sauce (if desired).
Combine lambic and cherries in a small saucepan. Bring to a boil. Reduce heat to medium and cook for 15-20 minutes, stirring frequently, or until thickened. Use wooden spoon to smash cherries against the side of the pan while it cooks for thicker sauce or puree with immersion blender after sauce has finished cooking for a smoother consistency.
After making this recipe, I discovered the best tool for cutting brownies: a bench scraper! Instead of cutting with a knife, use a scraper to cut perfect squares.