Preheat oven to 350 degrees. Grease and flour a 9x13 baking dish.
In a small saucepan, over medium heat, melt butter, and then add chocolate chips, vanilla, and brown sugar. Keep stirring with a silicone-covered whisk until melted and smooth. Take off heat.
Transfer the mixture into a large mixing bowl. Whisk slowly and let it cool for a couple of minutes.
Whisk in buttermilk until combined.
Whisk in eggs until combined and smooth.
Whisk in pumpkin puree until smooth.
Sift in flour, cocoa powder, pumpkin pie spice, baking powder, baking soda, and salt. Whisk until all smooth and all incorporated.
Pour mixture into the 9x13 baking dish, spreading evenly. Gently tap on the counter a couple of times to make sure it's spread evenly. Bake for 40-42 minutes.*Tip: bake brownies for about 5 less minutes if you want them gooey.
Make sure that brownies are completely cooled before adding frosting or you will melt the frosting.
Beat butter in a bowl of an electric mixer, on medium-high speed, for about a minute, until light and fluffy. Add cream cheese, and beat on medium-high speed, until smooth, 1–2 minutes.
Scrape bottom and sides of the bowl, and add pumpkin puree. Beat for a couple more minutes.
Lower speed to low and beat in powdered sugar and pumpkin pie spice.
Turn up the speed back to medium-high and beat 3–4 minutes, until the frosting is light, smooth, and fluffy.
Refrigerate 15–20 minutes before spreading over brownies.
Spread frosting all over cooled brownies.
Store brownies in the refrigerator because of the cream cheese frosting.