Preheat oven to 350°F. Sift the first 5 ingredients into a bowl. Stir in milk, melted butter, and vanilla. Pour the mixture into an 8 or 9-inch round or square pan. Mix together brown sugar and cocoa, then sprinkle it evenly over the top. Add hot water over the top, completely covering the brown sugar and cocoa. Carefully place the pan in the oven. Bake for 45 minutes. During baking, the cake will bubble and rise to the top of the pan, and a wonderful chocolate sauce will settle at the bottom. Remove the pan from the oven. Cool it slightly (about 10 minutes) and invert it onto a deep serving platter. Let it set up for about 15 minutes.