Preheat oven to 350°F (176°C). Line muffin pan with 12 liners.
In a small bowl, combine rice milk and vinegar, set aside.
Whisk together flour mix, cocoa powder, xanthan gum, baking soda, baking powder, and salt. Thoroughly whisk out any lumps in the cocoa powder. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, combine the rice milk mixture with the sugar, canola oil, and vanilla. Beat on medium speed until light and foamy, about 1 minute.
Using a sifter, sift in the flour mixture in three batches, mixing on low speed. Mix until smooth, about 30 seconds.
Fill the muffin liners about two-thirds full.
Bake in the center of the oven for 18 minutes, rotating the pan halfway through. Bake until a skewer inserted in the center of the cupcakes comes out clean. Let the cupcakes cool in the pan for 5 minutes, them transfer to a cooling rack to cool completely before frosting.
Make the chocolate buttercream frosting:
In the bowl of a stand mixer fitted with the paddle attachment, beat the shortening on medium speed until light and fluffy, about 2 minutes.
In a separate bowl, which together the confectioners sugar and cocoa powder, being sure to work out any lumps.
Combine the sugar mixture with the shortening, alternating with the rice milk. Add the vanilla and salt. Beat until smooth and fluffy, about 5 minutes. You can either pipe the frosting or spread it with a butter knife or frosting spatula, creating pretty swirls.