Preheat your oven to 350°F (175°C) and line an 8×8-inch pan with parchment paper.
In a large bowl, whisk together melted butter and sugar until combined.
Add eggs one at a time, whisking well after each, then mix in vanilla, buttermilk, and red coloring.
In a separate bowl, whisk flour, cocoa powder, and salt together.
Gently fold dry ingredients into wet until just combined. Don’t overmix.
If using, fold in the white chocolate chips.
In another bowl, beat cream cheese and sugar until fluffy (about 2 mins), then add yolk and vanilla.
Pour ¾ of the brownie batter into the prepared pan and spread evenly.
Dollop the cream cheese mixture on top, then spoon the remaining brownie batter over it.
Use a knife or toothpick to gently swirl the layers together for a marbled effect.
Bake for 25–30 minutes, or until a toothpick comes out with a few moist crumbs.
Let cool completely in the pan on a wire rack before lifting and cutting into squares.