Measure and combine flour, cocoa, and baking soda in a medium bowl using a fork.

Cream softened butter and sugar together for 5-6 minutes until the mixture lightens and sugar is fully incorporated.

In a separate bowl, whisk the egg and vanilla, then add to the butter-sugar mixture and mix on low speed until fully incorporated, scraping the bowl as needed.

Add the flour mixture and mix until just combined, then fold in the chocolate chunks to distribute them evenly throughout the dough; cover and chill the dough in the refrigerator for at least 3 hours or up to 5 days.

Preheat the oven to 350ºF and line two baking sheets with parchment paper.

Divide the chilled dough into 9 equal 1/4 cup portions, roll into balls, and arrange them with ample space on the prepared baking sheets, then gently flatten into thick, even disks.

Bake for 6 minutes, rotate the pans, and continue baking for another 5-6 minutes; remove, optionally sprinkle with flaky salt, and cool on the baking sheet for 10 minutes.