The key to smooth, creamy Cacio e Pepe is to go low and slow. Too high of a heat will cause the proteins in the cheese to seize up, leading to a stringy, clumpy sauce. It's important to add enough starchy pasta water to insulate the cheese from being overheated in the hot pan. If you're really struggling, you can place the pasta, starchy water, butter, and pepper in a double boiler (a large bowl set over a pot of boiling water) to heat it more gently. Toss constantly to heat evenly!