3/4 cup unsalted butter, cut into chunks
1/2 teaspoon salt
2/3 cup packed light brown sugar
1/4 teaspoon baking soda
1/2 cup granulated sugar
1 3/4 cups unbleached all-purpose white flour, plus more if needed
1 large egg, at room temperature
1 1/2 to 2 cups semisweet or bittersweet chocolate morsels, or substitute 11⁄2 to 2 cups chopped (1⁄8- to 1⁄4-inch pieces) semisweet or bittersweet (50 to 65 percent cacao) chocolate bars, disks, or pistoles
2 teaspoon vanilla extract
Position a rack in the middle of the oven; preheat to 350°F. Grease two large baking sheets or coat with nonstick spray; or line them with baking parchment.
In a large microwave-safe bowl with the microwave on 50 percent power, heat the butter until very soft but not completely melted or runny, stopping and stirring every 30 seconds.
Continue stirring until completely creamy-smooth and cooled to barely warm.
Vigorously stir the sugars into the butter, mashing out any lumps with the back of the spoon and stirring until well blended. Let cool to barely warm.
Vigorously stir in the egg, vanilla, salt, and baking soda until the mixture is well blended and smooth. Stir in the flour just until evenly incorporated.
Lightly fold in the chocolate morsels or chunks; excess stirring may cause melting. If the dough is too soft to shape, stir in up to 2 tablespoons more flour to stiffen it slightly.
On a sheet of wax paper with greased hands divide the dough into quarters. Then shape each into 6 or 7 balls, spacing 21⁄2 inches apart on the baking sheets. Pat down tops just slightly.
Bake (middle rack) one sheet at a time until lightly tinged with brown and are not quite firm when pressed in the middle, 10 to 13 minutes. Let stand to firm up, about 2 minutes. Using a wide spatula, transfer to wire racks. Cool completely.