In the bowl of a stand mixer fitted with the paddle attachment, or using an electric mixer and large bowl, beat the softened cream cheese and unsalted butter on medium speed until completely smooth and creamy.
Gradually add the powdered sugar, mixing on low speed at first to prevent splattering. Increase to medium speed and beat until fully incorporated and smooth.
Add the buttermilk and vanilla extract. Mix until thick, smooth, and pipeable. The frosting should be stiff enough to hold its shape.
Transfer the frosting to a pastry bag fitted with a large round piping tip.
Starting about 1 inch up from the outside bottom edge of a completely cooled bundt cake, pipe straight ribbons toward the center hole. Stop just inside the center opening. Rotate and repeat until the top of the cake is fully covered.