In a large bowl, whisk together the flour, cocoa powder, and salt. Set aside.
In a separate large bowl, beat ½ cup softened butter with a hand mixer or stand mixer for about 1 minute until smooth.
Add the granulated sugar and beat until well combined. Mix in the egg and 1 teaspoon vanilla extract.
With the mixer on low speed, gradually add the dry ingredients to the butter mixture. Mix just until a dough forms.
Cover and refrigerate the dough for 30–60 minutes.
Preheat the oven to 350°F. Line 2 baking sheets with parchment paper or silicone baking mats.
Roll approximately 2 teaspoons of dough into each ball and place on the prepared baking sheets. Gently flatten each ball. You should have approximately 48 cookies.
Bake for 6–9 minutes, or until the edges are crisp and the centers are set.
Let the cookies cool on the baking sheets for 5 minutes, then transfer them to a wire rack to cool completely.
To make the filling, beat the remaining ½ cup softened butter until smooth and creamy.
With the mixer on low speed, gradually add the powdered sugar and remaining 1 teaspoon vanilla extract. Once combined, beat on high speed for 1 minute until creamy. The filling will be thick.
Spread the cream filling onto the bottoms of 24 cooled cookies. Top with the remaining 24 cookies to make sandwiches.
Serve and enjoy.
Notes
These cookies will last for up to 1 week in an air tight container at room temperature.