Line baking sheets with parchment paper or silicone mats.

In a medium bowl, whisk together the flour, cocoa powder, salt, baking powder, and cornstarch. Set aside.

In a large mixing bowl, beat the butter and brown sugar on medium speed until light and fluffy, about 1–2 minutes.

Beat in the egg and egg yolk for 30 seconds, scraping down the sides as needed.

Gradually add the dry mixture to the wet ingredients, mixing on low speed until just combined. The dough will be very thick.

Scoop 1 tablespoon of dough (use a #60 cookie scoop) and roll into smooth balls. Roll each ball in granulated sugar and place 2 inches apart on prepared baking sheets.

Use the back of a teaspoon to press a divot into the center of each cookie before baking. Place the baking sheet in the freezer for 10 minutes to firm up the dough. You can freeze longer if needed.

Preheat the oven to 350°F (177°C). Bake for 10-11 minutes or until the edges are set but the centers still look soft. Remove from the oven and immediately use the teaspoon again to gently deepen the center indentation as it disappears while baking.

Allow cookies to cool completely before filling.

In a medium bowl, beat the peanut butter and butter with an electric mixer until smooth and creamy, 1 to 2 minutes. Add the powdered sugar, vanilla, and salt, then mix on low until a thick, dough-like filling forms.

Portion out 2 teaspoons of this mixture and add to the center of each thumbprint. Pro tip: The mixture will be sticky, so wet your fingertips, shake dry, and use your fingertips to press the mixture evenly into the cookies.

In a microwave safe bowl, combine the chocolate chips and coconut oil. Microwave for 1 minute on 50% power, stir, and return to the microwave for another minute on 50% power until melted.

Drizzle the chocolate over the peanut butter filling.