Preheat your oven to 350°F. Line two baking sheets with parchment paper or silicone baking mats.

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

In the bowl of a stand mixer fitted with the paddle attachment (or using a hand mixer), cream together the softened butter, brown sugar, and granulated sugar until smooth, scraping down the sides as needed. Beat in the egg and vanilla extract, then gradually add the dry ingredients, mixing until just combined. The dough will be sticky.

Scoop tablespoon-sized (or two-tablespoon-sized) balls of dough onto the prepared baking sheets, leaving space between each for spreading. Bake small cookies for 8-11 minutes, or larger ones for 12-15 minutes, until they just lose their glossy sheen. Let the cookies cool on the baking sheets for a few minutes before transferring to a wire rack to cool completely.

In a large bowl, beat the softened butter, peanut butter, and salt with a hand mixer until smooth, about 1 minute. Gradually add the powdered sugar, mixing until thick and smooth. Add the vanilla extract and 1 tablespoon of heavy cream, beating for 30 seconds. Add more cream, 1 tablespoon at a time, until the filling is spreadable but not runny (I usually use 2 tablespoons total).

Pipe or spread the filling onto the flat side of half of the cooled cookies. Top with the remaining cookies to form sandwiches. Store in an airtight container at room temperature for up to 3 days, or freeze for up to 1 month.