In the bowl of an electric mixer using paddle attachment (or using an electric hand mixer with a large bowl), beat together 8 oz cream cheese with 8 Tbsp butter on medium/high speed until creamy (3 min), scraping down the bowl as needed.
Sift in 3 cups powdered sugar with 1/2 cup cocoa powder to ensure there are no lumps then add 1/4 tsp salt. Mix on low speed until well combined, scrape down the bowl well then increase to medium/high speed and beat until smooth and whipped (1 min).
Add 1 tsp vanilla and beat on medium/high until smooth (1 min). Now it's ready to be piped onto cooled cupcakes.
Notes
It's best to pipe the frosting on after it is made. If you refrigerate the frosting, it will need a little time at room temp to soften and become pipe-able.