Preheat the oven to 350℉. Line an 8×2-inch round cake pan with a silicone liner. Set aside.
In a bowl, whisk the flour, cocoa powder, baking soda, and salt. In a large bowl, combine the melted butter and brown sugar with a rubber spatula. Add the eggs and vanilla, stirring until well blended. All at once, add the flour mixture and stir just until all the flour is moistened. Pour the hot water over the batter and stir just until it’s incorporated and the batter is smooth. Scrape the batter into the prepared pan.
Bake at 350℉. until a toothpick inserted in the center comes out clean, about 35 to 40 minutes. Let cool in the pan on a baking rack for 10 minutes. Run a thin knife around the edge and invert the cake. Turn it over again onto the rack and let cool completely.
Once the cake is cool, make the ganache: Put the chocolate in a medium heatproof bowl, metal or glass. In a small saucepan, bring the cream to a boil. Pour the hot cream over the chocolate and whisk gently until the chocolate is completely melted and smooth.
To frost cake: set the baking rack over a baking sheet or foil. Pour the warm ganache over the cake, letting drip over the sides. If necessary, use an icing spatula to spread it over the top of the cake and down the sides. Let set for about 1 - 2 hours before serving.