Melt half of the butter in a microwave-safe bowl. Add the remaining cut up butter to the hot melted butter and stir until completely melted. Let cool. Alternatively, you can melt the butter all at once and let it cool, but melting half of it and adding the remaining butter helps it cool faster.
Mix together the flour, baking soda and salt. Set aside.
In a mixing bowl, whisk together the cooled melted butter, brown sugar and granulated sugar. Add the egg, egg yolk and Mexican vanilla and whisk just until blended.
Add the flour mixture and stir to make a soft dough. Stir in bittersweet chocolate chips.
Using a heaping tablespoon, scoop up about 24 balls of dough and arrange them on two plates or one tray lined with wax, foil or parchment paper. Cover loosely and chill for several hours or until the dough balls are firm. When firm, you can bake as directed or put them in a freezer bag and freeze your individual balls of dough.
Preheat the oven to 325 degrees F. Arrange about half of the balls on one foil or parchment lined baking sheet spacing about 2 ½ inches apart. Set on the middle rack and bake for about 15-18 minutes or until cookies appear set and start to crack.
Let cool on the baking sheet for about five minutes, then carefully transfer to a wire rack and let cool completely.