Make sure all your cold ingredients are room temperature.
Melt both chocolates in 30-second intervals in the microwave, stirring in between, until smooth. Set aside.
In a large bowl, beat the cream cheese for 1–2 minutes until creamy.
In a separate small bowl, whisk the sugar and cocoa powder together, then beat it into the cream cheese until smooth.
Add eggs one at a time, mixing on medium speed until just incorporated. Do not overmix.
Scrape the bottom of the bowl often to avoid hidden cream cheese lumps.
Add the melted chocolate, vanilla, sour cream, corn starch, and salt. Mix until just combined.