Step 1In a small bowl, whisk together the buckwheat flour, salt and baking soda.
Step 2In a heatproof bowl, set on the grill over indirect heat, or in a saucepan over medium heat, heat the ½ cup butter, chocolate and canola oil until the butter and chocolate are melted and the mixture is smooth. Set aside to cool.
Step 3Beat the eggs, golden brown and granulated sugars and vanilla in with the chocolate mixture until thoroughly combined, then add the dry ingredients.
Step 4Grease a 10-inch cast iron or other heatproof skillet or the individual compartments of a cast iron muffin or mini cake pan, with melted salted butter. Add the batter (dividing it equally if baking smaller amounts) and leveling out as needed.
Step 5Bake the brownies in a closed grill over indirect heat (or in a 350-degree oven) until a cake tester comes out almost clean (it will have crumbs but should not be wet), about 30 minutes, depending on the size and shape of the brownies. Cool before removing from the pan.