Preheat oven to 350°F (180°C). Spray an 8x8-inch baking pan with nonstick cooking spray and set aside.
In a medium bowl, whisk together the cake flour, cocoa powder, baking soda, baking powder, and sea salt until combined. Set aside.
In a glass measuring cup, whisk together the buttermilk, vinegar, and vanilla until combined. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, or using a handheld mixer, beat together the butter and sugar until light and fluffy. Add the sour cream, egg, and red food coloring, if using, and beat until well combined. Don't worry if the mixture looks slightly curdled at this stage!
With the mixer running on low, gradually add the dry ingredients, alternating with the buttermilk mixture. Stop the mixer occasionally to scrape down the sides of the bowl.
Pour the batter into the prepared pan, and smooth out evenly with a spatula. Bake the cake for 28-30 minutes or until a toothpick inserted comes out clean and relatively crumb-free. Remove cake from oven and let cool completely to room temperature.
For the frosting: With a hand or stand mixer, cream together the butter and cream cheese until smooth. Add the vanilla bean paste and pinch of salt and beat until combined. Gradually begin adding the powdered sugar ½ cup (60 grams) at a time, stopping the mixer to scrape down the sides of the bowl. Spread the frosting evenly over the top of the cooled sheet cake. Top with any sprinkles of your choosing, if desired.