Lightly butter each ramekin, making sure to coat the bottom and sides. Sprinkle turbinado sugar into each ramekin, rolling it around to evenly coat. Shake out any excess. This creates a crisp, caramelized crust.

In a large bowl, beat the butter and cream cheese together until light and fluffy. Add the granulated sugar gradually and continue to beat until fully incorporated and the mixture is pale and smooth.

Beat in the eggs, one at a time, making sure each is fully mixed in before adding the next. Add the vanilla extract and mix again until the batter is smooth and glossy.

In a separate bowl, whisk together the flour and salt. Add the flour mixture to the batter in three parts, alternating with the milk, starting and ending with the flour. Mix just until combined—do not overmix.

Divide the batter evenly among the prepared ramekins. Place them on a baking sheet to make transferring easier. Bake in a preheated 325°F (160°C) oven for 35–40 minutes, or until the tops are golden and a toothpick inserted in the center comes out with just a few moist crumbs.

While the cakes are baking, prepare the sauce. In a small saucepan over low heat, whisk together the cream cheese, granulated sugar, and egg yolk. Stir constantly to prevent curdling. Once smooth and slightly thickened, remove from heat and stir in vanilla. Keep warm.

Once baked, let the cakes cool in the ramekins for about 5–10 minutes. Run a knife around the edges and carefully invert onto serving plates. Spoon the warm butter sauce over each cake and serve immediately.