Preheat the oven to 350F and butter a 9x13-inch baking pan or spray it with nonstick spray.
In a large bowl, beat the sugar, eggs, oil and vanilla for 3-4 minutes, until pale and creamy. In another bowl, stir together the barley flour, cocoa, baking powder, baking soda and salt.
Add the flour mixture to the sugar mixture in three parts, alternating with the buttermilk in two parts, stirring or beating on low speed just until combined after each addition. Scrape the batter into the prepared pan and bake for 35-40 minutes, until the top springs back when lightly touched.
Cool in the pan on a wire rack; once cool, frost it if you like.