Add the butter and sugar into the mixing bowl. Mix over medium-low speed for about 2-3 minutes, until light, fluffy, and well-combined (you can use a hand mixer or a stand mixer, if using a stand mixer use a paddle-shaped attachment).
Add the egg and vanilla extract, mix over medium-low speed for another 2-3 minutes, until well-combined. Scrape the sides of the bowl from time to time with a spatula.
Sift the flour, baking soda, and salt into the bowl. Gently mix until half combined.
Add about 3/4 of the chocolate chips (or roughly chopped chocolate) and roughly chopped pecans to the bowl. Mix until the chocolate and pecans are evenly distributed and all the ingredients are combined. Reserve the remaining nuts and chocolate to put on top of the cookie dough balls.
Cover the dough with plastic wrap, make an indentation in the center so that the dough can chill more evenly. Place the bowl in the fridge for 30 minutes. You can also bake the cookies right away but I prefer to chill the dough - the cookies will be slightly thicker. Preheat the oven to 350°F (180°C).
Scoop the dough with a 3-tablespoons-ice cream scoop onto a baking sheet lined with parchment paper (for best results - every dough ball should weigh about 40g/1.4oz). The dough balls should be spaced about 2 inches (5 cm) apart. Place 2-3 pieces of the remaining nuts and chocolate chips/chunks on top of of the cookie balls. Don´t flatten the dough. Put the dough with the remaining dough in the fridge while the first batch of cookies is baking.
Bake in batches for about 12 minutes if you're baking the cookies straight away or for 13 minutes if the cookie dough has been in the fridge for 30 minutes. The cookies are done when their edges are lightly golden. The cookies will be completely soft.
Take the baking tray out of the oven and let the cookies cool on the baking sheet (they will continue baking on the baking sheet). When they are holding their shape transfer them to a cooling rack to cool completely.
Enjoy!