Heat the oven to 350° F, first positioning a rack in the center of the oven. Sift together the flour, baking soda and salt and set aside.
Combine the brown sugar, coffee, butter and chocolate in a medium heavy saucepan over low heat. Cook, stirring occasionally until the butter and chocolate are almost completely melted. Remove from the heat and whisk until smooth. Whisk in the vanilla and set aside to cool.
When the chocolate mixture is about at room temperature, add the eggs and using a wooden spoon, stir well to combine. Add the dry ingredients and the milk in stages, alternating between the two and stirring well between each addition. (The batter should be fairly smooth, but it’s okay if you end up with some small lumps.)
Pour the batter into a greased pan measuring roughly 8 1/2” x 11” and bake for 25 to 30 minutes, until the center of the cake springs back when you press it lightly with your finger and a cake tester inserted into the middle of the cake comes out clean. Cool completely on a rack.
For the orange icing
Using a wooden spoon, combine the butter and confectioners’ sugar in a medium bowl. Stir in in the zest, orange extract, Cointreau and mascarpone, and then switch to a whisk to attack any lumps. Gradually whisk in the milk, adding just enough to make the icing spreadable.
Spread a thin layer of icing over the top of the cake and refrigerate briefly so that the icing firms up a little. Cut the cake into squares to serve.