• Toss the brown sugar and salt in a large mixing bowl and set aside. 

Brown the Butter

• Cut the butter into 1-inch pieces and place in a wide, light-colored saucepan so you can monitor the color as it browns. Melt over medium-high heat, swirling occasionally to keep the milk solids from settling. The butter will sizzle and foam; push the foam aside to check the color. Continue cooking and swirling until it turns a deep golden brown.
• Pour the browned butter over the brown sugar (scrape all the browned bits of milk solids into the bowl as well). Whisk the sugar mixture vigorously until well combined then set it aside while you make the tangzong. 

Make the Tangzhong

• Measure out 2-¾ cups of bread flour (330 grams) and place it in a bowl. You will use a portion of this measurement for the tangzhong and save the rest to finish making the cookie dough. 
• Place the milk in the saucepan that you used to brown the butter. Add 3 tablespoons of the bread flour and whisk well (it is ok if there are a few lumps). 
• Place the saucepan over low heat and cook, whisking constantly. As the mixture thickens, switch to a flexible spatula and stir until it comes together into a thick paste-like consistency. Transfer the tangzong to the bowl with the brown sugar and whisk well. 

Make the Cookie Dough 

• Add the cold eggs and vanilla and whisk until smooth. 
• Add the rest of the bread flour along with the baking soda and baking powder. Stir until well combined. If the mixture is warm, cover the bowl and place it in the fridge until it is chilled enough so the chocolate doesn’t melt when added. 
• Chop the chocolate into coarse pieces and stir it into the mixture. 
• Cover the bowl and chill the dough for 24 to 72 hours to firm up and develop flavor. (I prefer to scoop the dough into balls before chilling.) For large cookies, use an ice cream scoop to portion 85–90 g balls. For standard cookies, use a 2-tablespoon scoop to make 3-ounce (about 85 g) balls.

Bake the Cookies

• Preheat the oven to 350°F and line two baking sheets with parchment paper. 
• Arrange the balls of dough on the baking sheet, spacing them 3 to 4 inches apart.
• Wait until the oven is finished preheating then bake one pan of cookies at a time. Bake jumbo sized cookies for 18 to 22 minutes and standard sized cookies for 8 to 10 minutes. The edges should look set and browned. 
• Remove the cookies from the oven and let them cool on the baking sheet for 15 minutes.