Preheat oven to 350F (175C). Grease and flour a 9-inch round cake pan.
In a large bowl, whisk together gluten-free flour, cocoa powder, sugar, baking powder, baking soda, and salt.
In a separate bowl, combine buttermilk, oil, eggs, and vanilla extract.
Add the wet ingredients to the dry ingredients and mix until just combined. Be careful not to overmix.
Gradually add the boiling water to the batter and mix until smooth. The batter will be thin.
Pour batter into the prepared cake pan.
Bake for 30-35 minutes, or until a toothpick inserted into the center comes out with moist crumbs.
Let cake cool in the pan for 10 minutes before inverting it onto a wire rack to cool completely.
Once cooled, frost with chocolate frosting, if desired.
Optional Tips: For a richer chocolate flavor, substitute brewed coffee for the boiling water. If buttermilk is unavailable, substitute with regular milk mixed with 1 tablespoon of lemon juice or white vinegar, letting it sit for 5 minutes to curdle slightly. Ensure your gluten-free flour blend contains xanthan gum; if not, add teaspoon of xanthan gum to the dry ingredients.