This crust makes these easy to cut in squares. They will hold together very sell for serving to a crowd. Items cooked on a half sheet can be sliced into 4 rows by 6 rows to yield 24 servings. I generally go for the 24 portions if I’m cooking for groups of 150 or less.
Place the sugar and butter in a stand mixer fitted with the paddle attachment. (Alternatively, use an electric hand mixer and large bowl.) Beat on high speed until creamy and light in color, about 2 minutes. Scrape down the sides, add the eggs and vanilla and beat on low speed until incorporated.
Bake until the crust is dark brown and the marshmallows are golden-brown, 20 to 25 minutes. Cool for at least 1 hour in the pan before cutting into 24 bars for serving.