Preheat the oven to 350˚F. Line a large rimmed baking sheet with parchment paper.
Add the peanut butter to a large microwave safe bowl. Heat in the microwave for about 30 seconds, just to soften it slightly. Add brown sugar, egg and egg yolk, and vanilla extract and stir with a rubber spatula until smooth.
Add flour, baking soda and salt and use the spatula to knead the dry ingredients into the batter until incorporated.
Using a 1.5 tablespoon-size spring loaded cookie scoop, scoop 15 cookies on the prepared baking sheet about 2 inches apart. Use your hands to roll the dough into smooth flattened balls. Alternatively, you could roll the dough and use a cookie cutter to cut them into rounds.
Bake for 8 to 10 minutes, or until the edges are slightly golden brown. Remove from the oven and let cool on the sheet pan for 5 minutes, or until they are firm enough to handle. Then transfer to a cooling rack until completely cool, but save the baking sheet with parchment paper.
Dip the cooled cookies in the melted chocolate, covering each cookie about halfway. Place on the reserved baking sheet and let set in the fridge for about 10 to 15 minutes, or until the chocolate is firm to the touch.