Preheat oven to 350°. Butter a 9-inch round (2 inches deep) cake pan. Dust with flour and knock out excess.
Whisk together flour, cocoa, baking soda, and salt. Set aside.
Using an electric mixer on medium speed, beat butter and brown sugar until pale and fluffy. Add eggs, one at a time, beating well after each addition. Mix n vanilla.
Add flour mixture to butter mixture in batches, alternating with water. Begin and end with flour. Mix just until combined.
Pour batter into prepared pan. Bake 55-60 minutes, until cake is springy to the touch and a toothpick inserted in the center comes out clean.
Cool in pan for an hour. Invert onto rack, then onto serving plate. Dust with confectioners sugar before serving.