Preheat oven to 350°F (177°C).
Line a 9 x 13 inch cake pan with parchment paper.
Sift all dry ingredients (buckwheat flour, potato starch, sugar, cacao powder, salt, baking powder, xanthan gum) in a large bowl, and mix with a whisk. Create a well in the center of the dry mix.
In a large measuring cup, whisk together all wet ingredients (water, oil, vanilla extract). Add the wet ingredients into the dry and stir until combined. Batter should be quite thick.
Fold in chocolate chips.
Pour batter into cake pan lined with parchment paper. Use an offset spatula to spread the batter evenly.
Bake for 30 minutes. Test with a cake tester or toothpick to make sure it comes out mostly clean. If it needs to bake longer, rotate your cake pan and bake for another 5 minutes. Baking times may vary depending on your oven.
Cool completely before cutting. Brownies can be stored at room temperature for up to 3 days in an airtight container.