• Preheat oven to 350°F. Line a baking sheet with parchment paper. If you plan on chilling the dough, line 2 or 3 dinner plates with plastic wrap.
• Melt the butter and let it cool.
• While the butter is cooling, in a separate bowl whisk together flour, baking soda, salt, malted milk powder, and espresso powder.
• In a mixing bowl, stir together the cooled melted butter and both sugars.
• Add the egg yolk, the tablespoon of beaten egg, and vanilla. Stir until completely mixed. You can do this by hand or with the paddle attachment. If using the paddle attachment, don't overmix.
• Add flour mixture to wet and stir just until combined. The dough will be very thick. Fold in chocolate chips and toffee. This makes 24 oz of dough before add-ins, so if you want to make different flavors of cookies (based on chips), divide into parts and add other flavors. Caramel chips work well too.
• Scoop dough with a medium cookie scoop (about 2 tablespoons each) and space 2 inches apart on the sheet. If chilling the dough, scoop the balls onto dinner plates lined with plastic wrap, then cover and chill.
• Bake 11–13 minutes or until the edges are set and centers still look slightly underbaked.
• Cool on the sheet for 5 minutes, then slide the parchment onto the counter and let the cookies cool on the parchment paper. They’ll firm up as they cool.

Marshmallow Topping

• If you plan on adding the marshmallow topping, break or cut the marshmallows in half and press a chocolate chunk into the cut part. Squish as flat as you can, then put the marshmallows in the freezer for about 30 minutes. After about 10 to 11 minutes in the oven, pull the tray out and quickly put a frozen marshmallow piece in the center of each cookie. Continue baking just until cookies appear done (3 to 4 more minutes). Marshmallows should puff up and chocolate will soften. Let cool.