1. In a bowl, whisk together the flour, baking powder, and salt. Set aside.
2. In the bowl of a stand mixer (or with a bowl and a handheld electric mixer), cream the butter and sugar together. Add the egg and yolk one at a time, beating each in until well-incorporated and wiping down the bowl and beater as needed. Beat in the peanut butter until smoothly and completely incorporated. Beat in the vanilla extract.
3. Add the dry ingredients to the wet and mix in on lowest speed until mostly incorporated, with a few streaks of flour still visible. Finish folding the batter together by hand, just until evenly combined. Transfer the dough to an airtight container and chill in the refrigerator for at least 3 hours, or longer.*
4. Line a baking sheet with parchment paper. Portion the dough out into 1 1/4 ounces portions.** Roll each into a ball and place, about 6 at a time, on prepared baking sheet, well spread apart. Use your palm to flatten each ball out into a fairly round shape, about 3 1/2" wide and 3/16" thick. Use a 3 1/4 round cookie cutter to trim the edges off each round and form perfect circles. If desired, use the tip of a fork to press a pattern around the edge of each cookie. Chill another 15 minutes in refrigerator.
5. Meanwhile, preheat oven to 350ºF. Bake cookies in preheated oven for 6-8 minutes, just until tops look dry and cookies are puffy (they will flatten out as they cool).
6. Let cookies cool on pan for about 10 minutes before transferring to another surface to finish cooling. Cool completely before topping with ganache.

Salted Chocolate Ganache

1. Place chopped chocolate in a heat-safe bowl with room for cream.
2. In a separate, microwave-safe bowl, microwave the cream for 40-60 seconds, just until it starts to boil. Pour over the chopped chocolate and let sit for 5 minutes. Stir until smooth.
3. Use an icing spatula or the back of a spoon to spread a layer of ganache over each cookie. Once all have been covered and ganache has cooled a bit, sprinkle cookies lightly with sea salt. Cookies can be served immediately, or after ganache has set.
4. Store in an airtight container. These can be kept at room temperature for a day or two but otherwise should be kept in the refrigerator.