Preheat oven to 350 degrees F. Spray an 8" square or 9" round cake pan with nonstick spray or line a dozen muffin cups with cupcake liners.
In a medium size bowl, stir together the brown rice flour, xanthan gum, cocoa, brown sugar, chocolate chips (if using), nuts (if using), baking soda, and salt.
In a two cup glass measuring cup, measure the 1/2 cup oil, add the 1 cup of liquid, then add the vinegar and vanilla extract. Stir to combine.
Stir the liquid ingredients into the dry ingredients until just moistened.
Pour the batter into the prepared pan and bake for 30-35 minutes for cake or 25 minutes for cupcakes, or until the cake tests done. Set aside to cool.
For frosting, combine the chocolate chips and milk/water in a small saucepan. Heat over medium, stirring constantly until the chips are melted and the glaze is smooth. Spoon over the cooled cake and refrigerate until set.