Bring a pot of salted water to a boil and cook the shells according to their directions, shaving a minute off the cook time.
Heat a large pot over medium heat and add butter. Once sizzling, whisk in the flour to create a roux. Cook for 2 to 3 minutes, until slightly golden and fragrant. Very slowly stream in the milk and the cream while whisking the entire time. Cook for 5 to 6 minutes, stirring, until slightly thickened. Reduce the heat to low and add the goat cheese in 2 batches, stirring after each addition. Since it’s a little thicker, it will take longer to melt so continue to stir for 5 minutes or so after each addition.
Stir in the pecorino cheese and continue to stir until melted. Stir in the white pepper and garlic power. Taste and season additionally is desired – this is where you can add a little salt if needed! I like a touch of smoked sea salt too.
Immediately stir in the cooked pasta shells. Stir to coat completely. Serve right away – topping with chives for some color if you’d like. I like to top the shells with the mushrooms, but you can also stir them right in!