Heat the oven to 350° F.
Line an 8x8-inch metal baking pan with parchment paper.
Beat the butter and both sugars together for several minutes until they’re light and fluffy, either with an electric mixer or by hand with a wooden spoon in a large bowl. Beat in the egg and vanilla until they’re well combined. Separately, whisk together the flour, baking soda, and salt in a medium bowl. Add the dry ingredients to the wet ingredients and mix until they’re well combined. Fold in 1 cup of the chocolate chips.
Pour the dough into the prepared pan, spreading it to the edges and corners (it doesn’t have to be perfect—the dough will further spread in the oven). Bake until the bars are fully set and lightly browned on top, about 25 minutes. Allow them to cool completely in the pan on a cooling rack.
In a small microwave-safe bowl, heat the remaining chocolate chips in the microwave at 50% power in 30 second increments, stirring after each heating, until they’re fully melted (alternatively, you can melt the chocolate in a double-boiler on the stove). Drizzle the melted chocolate over the cooled cookie bars before cutting and serving.
BETTER YET: Cutting a pan of chocolate chip cookies into bars takes a whole lot less effort than scooping individual drop cookies!