Place butter in a large bowl and partially melt. There will be chunks, but it should be about 80% melted. Let butter come back to room temperature. If butter is still hot, it will melt the sugar and cause greasy cookies.
Add in brown sugar and granulated sugar. Whisk together until just combined and smooth.
Add in 1 egg and 1 egg yolk. (Discard the whites or save for another recipe.) Add vanilla extract and mix.
Add salt, baking soda, and flour (see note 1). Mix until just combined, being careful to not overmix.
Add in coarsely chopped Heath bars, 1 package of a coarsely chopped baking bar (save the other baking bar for step 9) or add 1 cup milk chocolate chips. If desired, add finely chopped pecans. Mix into the dough. Cover the bowl tightly and chill for 1–8 hours.
Preheat oven to 350°F. Line a sheet pan with parchment paper or a nonstick liner. Roll dough balls into tall cylindrical balls (see picture above for shape and size). If the dough gets too warm while rolling balls, place on a sheet pan and chill for 10–20 minutes in the fridge; we want the dough going into the oven to be not at all warm. Place 6–8 cookie balls on a sheet to give the cookies plenty of room to spread.
Bake 8–12 minutes, erring on the side of under-baking. The cookies will bake a little more out of the oven, so take them out as soon as the edges start to lightly brown.
Remove from oven, and if desired, press a few additional chocolate or Heath bar pieces into the top (to make them look pretty and ensure chocolate in every bite). Allow to cool on the pan for 5 minutes before transferring them to a cooling rack.
Optionally dip cookies in chocolate: coarsely chop the last baking bar and place the chopped chocolate in a microwave-safe bowl. Heat in bursts of 15 seconds, stirring between each burst for 15 seconds until melted. Drizzle melted chocolate over cookies or dip half of the cookie in the melted chocolate. Add a sprinkle of sea salt if desired. Let stand at room temperature to harden.
Notes
Note 1: When measuring flour, spoon it into the cup and level it. A full, not packed cup is key—spreading cookies mean too little flour, and thick, cakey ones mean too much.Storage: Store baked cookies in an airtight container for up to 5 days.Freeze dough balls on a baking sheet, then transfer to a freezer bag. Bake from frozen, adding 2-3 minutes to the time.