Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, leaving a 2-inch overhang on at least 2 of the sides to easily lift the bars out as a whole. Set aside.
In a large bowl, whisk together melted butter, brown sugar, and granulated sugar until smooth. Add in egg and vanilla extract. Mix until combined, scraping the bowl as needed.
Add the flour, baking soda, and salt, mixing until just combined.
Gently fold in chocolate chips. The dough will be thick and soft.
Transfer batter to the baking dish and gently spread to the edges.
Bake until edges are golden and the middle is set, about 25 minutes. Remove from the oven and immediately sprinkle with flaky salt, if using.
Once the bars have cooled, remove from the pan by lifting the parchment paper. Slice into 16 square bars and serve.