PREHEAT the oven to 350°F (175°C). Place the oven rack in the middle of the oven.
LINE a baking sheet with parchment paper (best choice) or silicone mat.
MEASURE the flour, baking soda and salt. Whisk to combine them.
CHOP the chocolate bar and the pecans.
CREAM the butter and the sugars together for 1½ minutes with an electric hand mixer. The correctly creamed mixture will be light and fluffy.
ADD the vanilla and the cold egg. Mix for 30 seconds.
ADD the dry ingredients all at once. Then slowly blend until everything is incorporated (30 to 45 seconds). Scrape the bottom and sides of the bowl.
POUR IN the chopped chocolate (and optional nuts). Stir them into the batter with a wooden spoon.
SCOOP the cookies dough with a teaspoon cookie scoop.
ROLL the cookie dough balls between your palms to round them and keep all the cookies a consistent shape.
SPRINKLE extra chocolate pieces to the tops of the cookie balls (optional).
PLACE the round cookie dough balls on the prepared baking sheet. Space the cookies 2 to 3 inches apart for teaspoon-sized balls.
BAKE the cookies for 9 to 11 minutes or until they are lightly browned. Optional: Rotate the baking sheet halfway through to ensure even baking. The edges should be set and turning golden brown.
REMOVE the cookies from the oven. Place the cookie sheet on a cooling rack for 5 to 10 minutes. Then transfer the cookies from the baking sheet to a cooling rack.
COOL the cookies completely. This may take up to an hour, depending on your kitchen's temperature.
STORE the cookies in an airtight container for 30 to 60 minutes. This is an optional step, but the cookie shape will solidify and the taste will mature.
SERVE with cold milk or hot chocolate. Or serve warm cookies with a scoop of vanilla ice cream.