Preheat the oven to 350 degrees F. Line baking sheets with silpat liners or parchment paper. Set aside.
For the brownie cookie batter, in a medium bowl with a handheld electric mixer (or in the bowl of an electric stand mixer), beat the butter, granulated sugar and brown sugar together until smooth and creamy, 1-2 minutes. Add the egg, yolk and vanilla and beat the mixture for 2-3 minutes until light in color.
In a separate small bowl, whisk together the flour, cocoa, soda and salt. Add the dry ingredients to the batter and mix until combined. Cover and refrigerate while making the chocolate chip cookie batter.
For the chocolate chip cookie batter, in the bowl of an electric stand mixer or with a handheld electric mixer, cream together the butter, granulated sugar, and brown sugar until smooth, 1-2 minutes.
Blend in the egg, egg yolk and vanilla, mixing for 2-3 minutes until the batter is very light in color. In a separate small bowl (you can use the same one as the brownie batter dry ingredients), whisk together the flour, baking soda, and salt. Add the dry ingredients to the batter with the chocolate chips and mix until no dry streaks remain and the chocolate chips are evenly distributed.
Portion both sets of dough into about 4 dozen equal pieces; they will be small teaspoon or so sized balls (if you want to be super precise, I weigh about .5 ounce balls for the brownie batter and .65 ounce balls for the chocolate chip cookie batter).
Grabbing one chocolate chip cookie ball and one brownie batter ball, press them together and use your hands to gently form into a cookie shape, flattening and turning to smooth the edges and form a flattish but still thick cookie shape; they'll spread out while baking. See the simple picture tutorial below the recipe, if needed.
Bake the cookies on the prepared baking sheets for 8-10 minutes. Don't overbake or they will be dry and crunchy - underbake just slightly for a soft, chewy texture. It's ok if the chocolate side crackles just a bit. Remove from the oven and let the cookies cool on the pan for 1-2 minutes before scooping onto a cooling rack to cool completely.
Revamped Recipe: as mentioned in the post above, I have completely revamped this recipe to make it foolproof thanks to many of you leaving feedback that the cookies weren't turning out right. I have also added weight measurements for those that like to be ultra-precise (let's start a support group together, shall we?). The cookies are still soft, chewy and completely delicious and dare I say even better than the first iteration of the recipe. Thanks for your patience and for all your comments to help me troubleshoot the recipe.Measuring: a note about measuring the dry ingredients - if you aren't using a scale to weigh the ingredients, measure with a light hand. I fluff the flour before scooping and leveling the cup but you may want to spoon in the flour/cocoa powder and leveling off that way. It doesn't take much flour to get five ounces in a cup which is what I use as my general standard and what I go by when developing a recipe.Flat Cookies: and last thing, if you generally end up with flat cookies (more flat than you want), it may be due to how you measure the flour (see the note above), overly soft butter (make sure it is just at room temperature) and/or high altitude (consider adding a tablespoon or two extra flour to the cookie doughs as that sometimes help high altitude cookie making). These cookies aren't overly thick but they shouldn't be super flat either - nice, soft and chewy, that's what you are after.