Preheat the oven to 350°F.
In a large bowl, whisk together the softened butter, brown sugar, and granulated sugar until well combined.
Whisk in the eggs and vanilla extract.
Add the flour, cocoa powder, salt, and baking soda (sift if clumpy) to the wet ingredients. Mix until just combined—do not overmix.
Gently fold in the semi-sweet chocolate chunks and milk chocolate chips using a spatula.
Pour the batter into a 9x13-inch baking pan sprayed with non-stick cooking spray. Smooth the top and sprinkle with half of the Halloween sprinkles.
Bake in the preheated oven for 50-60 minutes, or until a toothpick inserted into the center comes out clean.
Once out of the oven, add more sprinkles on top. Let the brownies cool for 30 minutes before cutting.
Enjoy your Halloween brownies! 🎃