1. Preheat your oven to 350 F (175 C). Grease and flour two 8-inch round cake pans with gluten-free flour. You can also line the bottoms with parchment paper for easier removal.

2. In a large mixing bowl, whisk together the gluten-free all-purpose flour blend, granulated sugar, unsweetened cocoa powder, baking soda, baking powder, and salt. Make sure there are no lumps and the ingredients are well combined.

3. In a separate medium bowl, whisk together the buttermilk, vegetable oil, eggs, and vanilla extract until smooth.

4. Pour the wet ingredient mixture into the dry ingredients. Mix on low speed with an electric mixer, or by hand with a whisk, until just combined. Be careful not to overmix.

5. Carefully pour the hot water or hot brewed coffee into the batter. Mix on low speed until the batter is smooth and thin. The batter will be quite liquidy, which is normal for this recipe and contributes to its moistness.

6. Divide the batter evenly between the two prepared cake pans. Bake for 30-35 minutes, or until a wooden skewer inserted into the center of the cakes comes out clean.

7. Remove the cakes from the oven and let them cool in the pans for 10-15 minutes. Then, invert them onto a wire rack to cool completely. Do not attempt to frost warm cakes, as the frosting will melt.

8. While the cakes cool, make the frosting. In a large bowl, beat the softened butter until creamy. Gradually add the sifted powdered sugar and cocoa powder, mixing until combined. Add the salt, vanilla extract, and milk/cream, beating until light and fluffy. Once the cakes are completely cool, spread a generous layer of frosting on top of one cake layer, place the second layer on top, and then frost the top and sides of the entire cake.