Preheat and prep: Heat your oven to 410°F (210°C). Line two baking sheets with parchment paper. This higher temperature helps set the outer crust fast while keeping the center soft.
Mix dry ingredients: In a bowl, whisk flour, baking powder, baking soda, salt, and cornstarch.
Set aside.
Cream the butter and sugars: In a stand mixer with the paddle attachment, beat the cold cubed butter with brown and granulated sugar on medium-high until light and slightly fluffy, 2–3 minutes. Cold butter is key for less spread and more height.
Add eggs and vanilla: Beat in eggs one at a time, then add vanilla. Mix just until combined.
Scrape the bowl as needed.
Add dry ingredients: Add the flour mixture and mix on low until the dough just comes together. Do not overmix.
Fold in mix-ins: Stir in chocolate chips/chunks and nuts, if using. The dough will be thick and chunky.
Shape tall mounds: Divide dough into 8–10 large portions (about 4–5 ounces/110–140 g each). Keep them rough and tall—don’t roll smooth balls.
Taller mounds help preserve that gooey center.
Optional chill: If your kitchen is warm or you want extra height, chill the dough mounds for 20–30 minutes. Not required, but helpful.
Bake: Place 4–5 cookies per sheet, leaving space. Bake 9–12 minutes, until the tops are just set with light golden edges.
The centers should look slightly underdone.
Set and serve: Let cookies rest on the sheet for 10 minutes to finish setting, then move to a rack. Enjoy warm while the chocolate is melty.