Preheat the oven to 350°F.Line the cookie sheets with a silicone baking mat or parchment paper.
Beat butter, peanut butter, brown sugar, egg, and vanilla extract together until completely combined.Add in almond flour, baking powder, and salt. Stir to combine.
Scoop dough onto the prepared cookie sheets using a cookie scoop.Using a fork, lightly press each cookie making a criss-cross pattern.Sprinkle with flaky sea salt if using.
Bake for 10-13 minutes.Let cool on the cookie sheet for 10 minutes before transferring to a cooling rack.Serve.
Store leftovers in an airtight container at room temperature for up to 4 days.