Preheat your oven to 350℉. Grease your baking dish and optionally line it with parchment paper to make the brownies easier to remove.
In a mixing bowl, combine your flour, cocoa powder, corn starch, and baking soda. Set aside.
In your stand mixer, cream together the butter and sugars until the mixture is light and fluffy.
Add in your one full egg and vanilla, and mix more.
Continue to mix on low, and add in ⅓ of the dry mix. Mix, then add in half the milk. Mix, then add in another ⅓ of the flour, then the remaining milk, then the remaining flour. Try not to over-mix this at the end, you don't want to whip air into the batter because you're trying to keep it dense and fudgy.
Pour your batter into your baking sheet. Spread it evenly with your spatula if need be.
Bake 20-22 minutes. It will look undercooked, but will set as it cools. Let it fully cool in the pan.
Make your ganache. This is super easy. Heat up some water in your sauce pan or dutch oven. In a mixing bowl on top of this sauce pan, add your heavy cream and chocolate. Stir it and let the heat gradually melt the chocolate, until it's all combined.
Spread this ganache over your cooled brownies, and decorate with any sprinkles you desire.
Chill in the fridge and serve once cold, to keep it nicely set.