Preheat oven to 350ºF. Beat peanut butter and butter with sugars on high speed in a stand mixer for about 3 minutes, until nicely creamed. Beat in yogurt, egg, and vanilla.
With mixer speed on low, sprinkle in the flour, then the baking soda and salt. Beat to combine.
Stir in crisp rice cereal. Drop cookies by tablespoonfuls onto parchment-lined baking sheets about 2 inches apart. If desired, use a 1/2-teaspoon measure to make small indents in the cookies, and spoon in about 1/4 teaspoon more peanut butter per cookie. Sprinkle with a few grains of crisp rice.
Bake for 6 minutes, rotate the pan, and bake for 2-3 minutes more, until golden but still slightly underdone. Sprinkle with flaky salt, if desired.