1. Make the cookies: Preheat the oven to 325˚. In the bowl of a standing mixer outfitted with the paddle attachment, beat the butter, brown sugar, granulated sugar and cocoa powder together at medium-high speed until light and fluffy, about 2 minutes. Add the melted chocolate and vanilla extract and beat until combined.

2. In a bowl, whisk the flour with the baking soda and salt. Gradually add the dry ingredients to the butter mixture and beat at low speed until incorporated.

3. Turn the dough out onto a lightly floured work surface and, using a floured rolling pin, roll it out until it is ⅛-inch thick. Using a 4-by-2-inch pig-shaped cookie cutter, punch out the cookies. Reroll the scraps and cut the rest of the cookies. (The dough may also be cut into 2-inch squares.)

4. Arrange the cookies on two parchment- or silicone-mat-lined baking sheets and bake for 12 to 15 minutes, until the cookies are set at the edges. Transfer the baking sheets to a cooling rack and let cool completely.

5. While the cookies cool, make the filling: Add the bacon fat, salt, vanilla seeds and brown sugar to a small saucepan. Cook over medium-low heat, stirring occasionally, for 5 minutes. Strain the mixture into a jar, cover and refrigerate until solid, about 2 hours.

6. After the bacon mixture has chilled, add it to the work bowl of a stand mixer outfitted with the whisk attachment and beat on medium speed until fluffy, about 30 seconds. Slowly add the confectioners' sugar and continue beating until fully incorporated. Add the milk and beat until the filling is the consistency of thick cake frosting.

7. To assemble the pigwiches, spread about 2 tablespoons of the filling on one cookie and top with another cookie; press lightly to seal. Repeat with the remaining cookies and filling. Serve immediately or store in a resealable container for up to one week.