Prep. Preheat the oven to 325°F. Line an 8x8 or 9x9" pan with parchment paper, leaving an overhang for easy removal.
Cream. In a large bowl, cream together ½ cup butter, ½ cup brown sugar, and ⅓ cup granulated sugar for 1 minute. Add 1 egg and 1 ½ teaspoons vanilla and mix until combined.
Add dry ingredients. Mix in 1 ½ cups flour, ½ teaspoon baking soda, and ½ teaspoon salt just until combined. Fold in ¾ cup semi-sweet chocolate chips and ½ cup milk chocolate chips, saving some for the top.
Press. Scoop the dough into the pan and press down evenly. Place the reserved chocolate chips on top.
Bake. Bake for 18 to 25 minutes, until the edges are golden and the center still looks slightly underbaked. Sprinkle with flaky sea salt if desired.
Cool. Let cool completely, then lift from the pan and slice.
Notes
Storage: Once completely cooled, store in an airtight container at room temperature for up to 3 days.To freeze, wrap whole or in individual bars in plastic wrap, then foil to prevent freezer burn, and freeze up to 3 months. Thaw at room temperature about an hour before serving, or warm in the microwave for 10 to 15 seconds.Chocolate chips: If you'd like you can swap in chocolate chunks, peanut butter chips, white chocolate chips or m&m's for half the mixture.