Position racks in the lower and upper third of the oven and preheat to 325 degrees F. Line 3 baking sheets with parchment or silicone mats. (If you don't have 3 pans, simply cool the pan between batches.)
Put the butter and the unsweetened and semisweet chocolates in a medium microwave-safe bowl. Heat at 75 percent power in the microwave until soft, about 2 minutes. Stir and heat again until melted, up to 2 minutes more. (Alternatively, put the chocolates and butter in a heatproof bowl. Bring a saucepan filled with an inch or so of water to a very slow simmer; set the bowl over, but not touching the water, and stir occasionally until melted and smooth.)
Stir the light brown and granulated sugars and vanilla into the chocolate mixture with a wooden spoon. Add the eggs and buttermilk and beat vigorously until thick and glossy.
In another bowl, whisk the flour, cocoa, cinnamon and salt together. Add the dry ingredients to the wet and stir until just mixed. Stir in chocolate chunks and dried cherries, if using.
Drop the batter in heaping tablespoons onto baking sheets--a small ice cream scoop is ideal for this. Space the cookies about 2-inches apart. Bake until the cookies set but are soft and fudgy on the inside, 12 to 15 minutes.
Cool cookies on the baking sheet for 5 minutes, then transfer to a rack to cool completely. Serve.
Store cookies in a tightly sealed container at room temperature for up to a week.