Preheat the oven to 350 degrees F (175 degrees C). Grease two 8-inch cake pans and dust with cocoa powder or spray with baking spray.

Sift flour, baking powder, baking soda, and salt for cake together in a bowl.
Dissolve cocoa and brown sugar in the boiling water.

Beat butter and sugar together in a separate bowl until creamy. Add vanilla extract. Add of the flour mixture and mix; add 1 egg and mix. Add remaining flour mixture and combine. Mix in remaining egg. Pour in cocoa mixture gradually and mix well until combined. Divide batter evenly between the prepared pans.

Bake in the preheated oven until a toothpick inserted into the center of each cake comes out clean, 18 to 20 minutes. Let cakes cool completely, about 1 hour.

Meanwhile, bring cream to a boil in a saucepan and add vanilla extract. Place dark chocolate in a bowl and pour hot cream on top. Stir until chocolate is melted and a smooth mixture forms. Add cinnamon, and salt; mix. Let chill for 30 minutes.

Spread some ganache on top of the cooled cakes and layer them. Spread remaining ganache over the whole cake and smooth out with a spatula. Chill for 30 minutes.