Preheat the oven to 350 F. Spray a spool cake pan or muffin pan with nonstick cooking spray.
Whisk together the flour, cocoa, and salt in a small bowl, and set aside.
Combine the butter, both chopped chocolates, and coffee or water in a large microwave-safe bowl. Microwave in 30-second increments, stirring after every 30 seconds, until the butter and chocolates are melted and the mixture is smooth.
Add the granulated sugar, and whisk it in. Whisk in the eggs one at a time, whisking very well after each addition. It should go from a grainy mess to a shiny mixture with the texture and appearance of chocolate pudding.
Add the flour to the bowl, and gently stir it in with a spatula. Use a level cookie scoop to fill each cavity in the pan—don’t overfill, since you only want these brownie layers to be about ½-inch tall. Bake them for 9-10 minutes, until a toothpick inserted into the center comes out with just a few moist crumbs attached. (If you are using a muffin pan, you may need to adjust the batter amount and baking time.) If the brownies have domed, press down on the tops gently to level them out.
Let them cool for 5-10 minutes, then gently remove them from the pan and cool completely on a wire rack. Repeat until all of the brownie layers are baked. Cool completely before assembling.
To Make the Frosting and Decorate:
Place the chopped chocolate in a medium bowl, and pour the cream into a small saucepan. Heat the cream over medium heat until it comes to a simmer and bubble appear all along the edges of the pan. Pour the hot cream over the chopped chocolate and let it sit for one minute to soften the chocolate.
Gently whisk the chocolate and cream together until the chocolate is completely melted and the mixture is smooth and shiny. This is your ganache.
Press a layer of cling wrap directly on top of the ganache, and refrigerate it until it is thick enough to spread and has the texture of peanut butter—about 45 minutes. Alternately, if you make this in advance, you can leave it at room temperature for a day or so and it will cool and thicken up by itself.
To assemble, spread a tablespoon of ganache on top of one brownie layer, and spread a second tablespoon of ganache on top of a second brownie layer. Stack one layer on top of the other, and sprinkle a big pinch of rainbow chips on top of the brownie cake. Repeat until all of the cakes are assembled. If the cakes seem a little unstable, refrigerate them briefly to set the chocolate frosting a bit more. These Cosmic Brownie Mini Cakes are best served at room temperature.