1 c (142 g) flour
3/4 tsp baking powder
1/2 tsp baking soda
1/4 tsp salt
14 Tbsp (199 g) butter, softened
1 c (199 g) sugar
1/4 c (40 g) packed light brown sugar
1 tsp vanilla
2 1/2 c (225 g) old-fashioned rolled oats
6 oz (170 g) good-quality white chocolate bar, chopped (Deb of Smitten Kitchen recommends not using chips, but they're all I had and I happen to like how they taste!)
1/2 tsp flaky sea salt (like Maldon or fleur de sel), for sprinkling
Preheat your oven to 350F. Line two baking sheets with parchment paper and set aside.
Combine the flour, baking powder, baking soda, and salt in a medium bowl and set aside.
In a large bowl, beat the butter and sugars until light and fluffy, about 5 minutes. Scrape down the sides of the bowl, then add egg and vanilla and beat until incorporated, about 3 minutes. Scrape down the bowl again and add half of the flour mixture. Mix to combine, scrape down bowl, and add remaining half. Mix until just incorporated, then add the oats and chocolate and mix until evenly distributed.
Using a 2 Tbsp scoop, portion the dough into as many balls as you can. Roll into a sphere shape between your palms and place on lined sheets about 2 1/2" apart. Gently press each ball into a 3/4" thickness and sprinkle with a few flakes of salt. Bake until cookies are golden brown, 13 to 16 minutes, rotating pan halfway through. Transfer sheet to a cooling rack and allow to cool completely.