Heat the oven to 410°F (210°C) and line baking sheets with parchment paper.
Mix Butter and Sugars – Beat cold, cubed butter with granulated and brown sugar until combined; the mixture will be grainy, not fluffy.
Add Eggs and Vanilla – Mix in eggs one at a time, then stir in vanilla extract.
Combine Dry Ingredients – In a separate bowl, whisk flour, baking powder, baking soda, and salt. Gradually mix into the wet ingredients until just combined.
Fold in Chocolate and Walnuts – Stir in chocolate chips and walnuts (if using). The dough will be thick and dense.
Portion the Dough – Use an ice cream scoop to form large dough balls, spacing them apart on the baking sheets.
Bake – Bake for 10-12 minutes until edges are golden but centers remain soft and slightly underbaked.
Cool and Enjoy – Let cookies cool on the baking sheet for 10 minutes, then transfer to a wire rack. Serve warm for the best gooey texture.