Add the cream cheese to the bowl of a stand mixer fitted with the paddle attachment and mix on low speed until broken down. Scrape down the sides.
32 ounces full-fat cream cheese
Mix together the sugar and cornstarch in a bowl, then add it to the cream cheese and mix on low until incorporated. Scrape down the sides again.
1⅓ cups granulated sugar, 3 tablespoons cornstarch
Whisk together the eggs and vanilla, then drizzle it into the cream cheese mixture (with the mixer running on low speed) in four batches until fully incorporated.
4 large eggs, 2 teaspoons vanilla paste or extract
Scrape down the sides once more, then add the sour cream and mix on low speed until combined.
1 cup full-fat sour cream
Add half of the cheesecake batter to another bowl and set it aside.
Chop the chocolate and add it to a heatproof bowl. Microwave the chocolate in thirty second bursts, stirring in between each round, until fully melted. Allow it to cool for about 5 minutes.
4 ounces semi-sweet chocolate bar (60-70% cacao)
Add the melted chocolate to one bowl of batter and use a rubber spatula to fold it in until the chocolate is fully incorporated into a smooth, chocolate batter.