Preheat the oven to 350F and line a 9x13-inch pan with parchment or spray it with nonstick spray.
In a large bowl, whisk together the flour, brown sugar, cocoa, baking powder, baking soda and salt. Pour in the oil and/or melted butter, eggs, buttermilk, coffee and vanilla and whisk until smooth.
Pour into the prepared pan and bake for 30-35 minutes, until the cake is springy to the touch. Let cool completely before frosting.
To make the frosting, melt the chocolate chips in the microwave or over low heat on the stovetop, stirring frequently until melted and smooth. Whisk in the sour cream and vanilla, if you're using it. Spread over the cooled cake. (If it's too thin, refrigerate until it firms up a bit.) Sprinkle with sprinkles right away, if you're using them. Serves 12 to 20.