Oil a square baking dish (mine was approximately 8-1/4 x 8-1/4 x2-1/3-inches); set aside.
In a large bowl, whisk together the milk, oil, margarine, vanilla, brown sugar, and granulated sugar. (You'll end up with tiny bits of margarine floating around. You want that. Don't let them melt into the liquid.) Set aside.
In a separate bowl, whisk together the potato starch, brown rice flour,white rice flour, xanthan gum, cocoa powder, baking powder, and salt. Sift intothe wet mixture. Whisk until smooth.
Fold in the nuts and chocolate chips. Pour into the prepared dish.
Bake in the microwave on high for 6 minutes. Check to make sure that the entire surface is firm to the touch including the center. If not, microwave in 30-second intervals, testing after each interval for doneness, until done. (I baked mine for a total of 8 minutes.)
Transfer to a wire rack to cool completely. (Important! If this step is not done, the center's bottom will steam up and appear unbaked.)
*For vegan/organic sugar, please be sure to break down those large crystals prior to adding the margarine/buttery spread.
https://glutenfreerecipebox.com/gluten-free-microwave-brownies-full-batch/