Grease and line one 6-inch cake pan (with straight sides) with parchment paper as per this tutorial. Set aside.

Preheat the oven to 325 degrees F.

In a medium-large mixing bowl, sift together the gluten-free flour, sugar, baking soda and salt. Make a well in the center and set aside.

In a small measuring pitcher, mix together the vinegar and milk; set aside.

In a small-medium saucepan, combine the butter, water, and cocoa powder. Place on the stove over medium heat and cook just until the butter is completely melted.

Pour the hot butter/chocolate mixture into the well of your gluten-free flour mixture and beat (by hand or with a an electric mixer) until smooth and well combined. Add the soured milk, egg, and vanilla, and beat until smooth.

Pour the batter into the prepared cake pan. Bake for 1 hour or until a toothpick inserted into the center of the cakes comes out clean.

Allow the cake to cool in the pan for 10 minutes before turning out onto a wire rack. Cool completely before decorating.

Trim the rounded top off of the cooled cake, and use a long serrated knife to split the cake into 2 equal layers. Frost with your desired gluten-free frosting or whipped cream.