Preheat the oven to 350ºF and line a large baking sheet with parchment paper. (Prepare two pans if you'd like to bake the whole batch at once.)
In a large bowl, combine the softened butter and sugar and mix well. Add in the egg and vanilla, then stir again.
Add in the buckwheat flour, cocoa powder, baking soda, and salt. Stir well, until a thick cookie dough is formed. Fold in the chocolate chips, reserving a few to press into the top of the cookies.
Use a tablespoon or 1-ounce cookie scoop to scoop the dough onto the prepared baking sheet. Press the reserved chocolate chips into the top of the cookies and bake at 350ºF for 9 to 10 minutes. The cookies should spread slightly as they bake.
Let the cookies cool on the pan for at least 20 minutes, then they can be enjoyed warm for a brownie-like experience. Or transfer them to a wire rack to cool completely. These cookies will keep well in an airtight container in the fridge for up to 5 days if you don't eat them all before then.
Notes
Nutrition information is for 1 cookie, assuming you get 24 from this batch. This information is automatically calculated and is just an estimate, not a guarantee.Buckwheat Note: Use light-colored buckwheat flour (made with hulled buckwheat groats) for the best flavor. If the buckwheat looks gray or blue when mixed, it will have a more bitter flavor due to the hulls. Make homemade buckwheat flour or buy Anthony's Organic Buckwheat Flour from Amazon (affiliate link), which is what I used for testing.In my opinion, salted butter makes cookies taste butter than using unsalted butter. But feel free to use what you have on hand. Make it dairy-free: Swap the butter for vegan butter or refined coconut oil. (Refined coconut oil will not add a coconut flavor to the cookies.)Make it vegan: Replace the dairy as directed above, and use a flax egg as an egg substitute. Bake the cookies for at least 10 minutes in this case, since they will be slightly softer with these substitutions.