In a large bowl, using a handheld or stand mixer, beat the butter, granulated sugar and brown sugar together on medium-high speed until fluffy and light in color, about 2–3 minutes. Add egg and vanilla extract, then beat on high speed until combined.

The marshmallows on top of the "brownie-like" cookies are "toasted to perfection" under the broiler, according to cookbook author Sarah Fennel

I tried hot chocolate recipes from chefs Ina Garten, Jacques Torres, Sandra Lee, Alton Brown, and Carla Hall and reviewed each to find the best one.