In a bowl, whisk together the flour, cocoa powder, baking soda, and salt.
In a large bowl, using a spatula or wooden spoon, stir and combine the melted butter and brown sugar.
Add the eggs and vanilla and stir until well blended.
Add the flour mixture and stir just until all the flour is moistened.
Pour the hot water over the batter; stir just until it’s incorporated and the batter is smooth. Scrape the batter into the prepared pan.
Bake until a toothpick inserted in the center comes out clean.
30 minutes for a 9-inch pan
35 to 40 min. for an 8-inch pan.
Let cool in the pan on a rack for 10 min.
Run a knife around the edge and invert the cake (peel off the parchment if necessary). Invert it again onto the rack and let cool completely.
Once cool, set the rack over a baking sheet or foil.