Preheat the oven to 350°F and line baking sheets with parchment paper.

Beat the softened butter, granulated sugar and brown sugar together for about 3 minutes until creamy and fluffy.

Add the eggs one at a time, followed by the vanilla.

Mix in the sour cream.

In a separate bowl, whisk together the flour, cocoa powder, cornstarch, baking powder, baking soda and salt.

Gradually add the dry ingredients to the wet ingredients and mix just until combined.

Chill the dough for 30 minutes.

Scoop approximately 1/3 cup of dough per cookie.

Roll gently into balls and place several inches apart on your prepared baking sheets.

Lightly press each ball into a thick disc.

Bake for approximately 11–13 minutes. The edges should look set while the centers remain soft.

Don't wait until the entire cookie looks completely baked—the cookies will continue setting as they cool.

Allow them to cool on the baking sheet for about 5 minutes before transferring to a wire rack.

Cool completely before adding the peanut butter buttercream.

Beat the butter and peanut butter together for 3–4 minutes until smooth and fluffy.

Gradually add the powdered sugar.

Add the vanilla and 3 tablespoons cream.

Beat for another 2–3 minutes until light and creamy.

Add additional cream, 1 tablespoon at a time, until you reach a soft, easily spreadable consistency.

Once the cookies are completely cool, add a generous scoop of peanut butter buttercream to the center of each cookie.

Use a small offset spatula to spread the frosting almost to the edges, leaving some of the chocolate cookie visible around the outside