Preheat oven to 350°F/175°C. Grease a 9x13 inch pan and line with parchment paper.
Sift the flour, cocoa powder, sugar, baking soda, baking powder and salt together in a medium bowl.
In a large bowl, whisk the buttermilk, oil, eggs and vanilla. Whisk in the dry ingredients until combined. Pour in the hot coffee and continue mixing until smooth.
Pour the batter into the pan and bake for 35-40 minutes. A toothpick inserted should come out with a few crumbs.
Tahini Buttercream
In a mixer (or by hand!) using a paddle attachment, beat the butter and tahini until completely smooth.
Add the icing sugar, salt and vanilla. Beat for another 2-3 minutes and then top cake.