Preheat oven to 350°F (175°C). Butter a 9-inch round (2 inches deep) cake pan. Dust with flour and knock out the excess.
Whisk together the flour, cocoa, baking soda, and salt. Set aside.
Using an electric mixer on medium speed, beat the butter and brown sugar until pale and fluffy. Add the eggs, one at a time, beating well after each addition. Mix in the vanilla.
Add the flour mixture to butter mixture in batches, alternating with water. Begin and end with flour. Mix just until combined.
Pour the batter into the prepared pan. Bake 55-60 minutes, until the cake is springy to the touch and a toothpick inserted into the center comes out clean.
Cool in the pan on a wire rack for an hour. Then invert onto a wire rack, then onto serving plate. Dust with confectioners' sugar before serving.
How to store: You can store this cocoa cake wrapped or in an airtight storage container at room temperature for 3 to 4 days. Freeze the cake for up to 3 months; you can wrap the whole thing or individual slices, then store it in an airtight container or freezer bag. Let it thaw at room temperature or warm in the microwave before serving.