Line an 8×8” baking pan with 2 sheets of overlapping 8”-wide parchment paper. Parchment should be long enough to fold up the side of pan and provide an easy handle for removing bars after baking. To preheat oven, press “Bake” and use “+/-” to set temperature to 350°F (default temperature). Press “Start” to confirm temperature, then use “+/-” or “+5 MIN” to set bake time for 20 minutes.
Whisk together flour, baking powder and salt in medium-size bowl. In a separate larger bowl, whisk together melted butter and sugar until smooth. Add egg and vanilla and continue to whisk until smooth and creamy, about 1 minute. Fold in flour mixture with 1/2 cup chocolate chips. Stir until just combined and no dry spots remain.
Transfer dough to prepared baking pan and spread into even layer. Scatter remaining chocolate chips on top and place pan into oven. Press “Start” to begin baking.
Remove cookie bars from oven and sprinkle with flaky sea salt. Let cool in pan 5 minutes, then use parchment paper overhang to carefully lift out of pan. Cut into squares and serve warm.