In a medium bowl, whisk together the flour, oats, baking powder, baking soda, and salt.

In a large bowl, whisk the melted butter, sugar, and vanilla until smooth and homogeneous. Add the egg and whisk until fully combined and creamy. Add the dry ingredients and use a flexible spatula to stir until almost no flour is visible. Add the dates and stir until completely combined.

Line a baking sheet with parchment. Scoop the dough into eight 2" balls (about 80g each; a level scone and muffin scoop works well here) and place on the prepared baking sheet. Cover and refrigerate for at least 1 hour and up to 2 days.

Preheat the oven to 375°F with a rack in the upper third. Shape the dough mounds into neater balls by rolling between the palms of your hands. Return to the baking sheet, spacing them evenly.

Bake the cookies for 13 to 16 minutes, until puffed, with golden-brown exteriors. Remove the baking sheet from the oven and immediately use a flat spatula to press each cookie flat, eliminating any puffing.

Return the cookies to the oven, turn the oven off, and leave inside for 4 to 5 minutes, until the cookies are slightly darker and have puffed back up.

Remove the baking sheet from the oven, press the cookies flat again, then sprinkle with flaky salt. Let the cookies cool completely on the baking sheet before serving.

Store oatmeal-date cookies airtight at room temperature for up to 5 days; freeze for longer storage.