Preheat your oven to 325°F (163°C). Grease the sides and bottom of an 9x13 inch baking pan with cooking spray or line it with parchment paper, leaving some overhang for easy removal. You can use metal binder clips to secure the parchment paper to prevent it from sliding around.
In a large microwave-safe bowl, melt the unsalted butter and both sugars in 30-second increments until the butter dissolves into the sugars, whisking after each increment. This will take about 3-4 increments.
Whisk in the eggs one at a time, whisking after each addition. Add the vanilla extract and mix until combined.
Using a rubber spatula, fold in the dark cocoa powder, all-purpose flour, and salt into the wet ingredients until just combined. Be careful not to overmix.
Pour the brownie batter into the prepared baking pan, spreading it evenly with a spatula.
Bake in the preheated oven for 35-40 minutes, or until the edges are set and a toothpick inserted into the center comes out with moist crumbs (not wet batter).
Allow the brownies to cool to room temperature before adding the ganache, about 30 minutes.
For the Ganache
Place the heavy cream in a medium microwave-safe bowl and heat until it's hot and slightly bubbly to melt the chocolate. Start with 30 seconds and check, adjusting as needed depending on your microwave. Remove from the microwave and add the chocolate. Let it sit for 5 minutes untouched.
After 5 minutes, stir until everything melts and the mixture is smooth. If you have a chunky ganache, you can microwave it in 20-second increments and stir until smooth.
Using the back of a spoon or a small offset spatula, spread the chocolate ganache evenly over the cooled brownies.
Let the ganache set up for a few hours before cutting and serving the brownies, or place them in the fridge until the ganache sets up.