Preheat the oven to 350 degrees F and line a cookie sheet with parchment paper, then set aside.

In a medium-sized bowl, whisk together the flour, baking soda, and cornstarch. Set aside.

In a separate large bowl using an electric mixer or a stand mixer fit with the paddle attachment, cream together the butter and brown sugar. Mix for about 2 minutes or until light and fluffy with no lumps.

Add the peanut butter to the sugar and butter mixture and continue to cream together until smooth. Use a silicone spatula to scrape down the sides of the bowl as you go.

Add the egg and vanilla to the butter mixture and continue to mix until well combined.

Add the dry ingredients to the wet and mix just until a nice, soft dough forms. Scrape down the sides of the bowl one last time to ensure that everything has been incorporated evenly.

Scoop the dough into about 1 Tbsp. sized pieces. Roll each piece into a ball. Next, roll each dough ball into a log about 3" long. Use the palm of your hand to flatten each log into a long disk shape. Pinch the middle sides of the log to create the peanut shape.

Once the peanut shapes are ready, place the cookie onto the prepared cookie sheet, then use a fork to create score marks evenly up and down the cookie.

Repeat these steps for each cookie. Place each cookie about 2" apart on the prepared cookie sheet, then bake for 9-10 minutes or until golden brown on the edges.

Allow the cookies to cool completely before filling.

To make the peanut butter filling, cream together the peanut butter, confectioner's sugar, heavy cream, and vanilla. Cream until the mixture is nice and smooth and there are no lumps. Add more heavy cream if your mixture is too thick (this may vary depending on peanut butter brands).

Transfer the filling to a piping bag and fill the bottom side of one cookie at a time, then top with another cookie, facing up. Repeat this process for each cookie.