Whisk together flour, cocoa powder, and salt in a medium bowl.

In a large bowl, cream 1 cup butter and granulated sugar until light and fluffy.

Beat in the egg and vanilla extract until well combined.

Gradually add the dry ingredients to the wet ingredients, mixing until a dough forms.

Divide dough in half, wrap in plastic, and refrigerate for 30 minutes.

Preheat oven to 350°F (175°C) and line baking sheets with parchment paper.

Roll out dough to 1/8 inch thickness on a floured surface and cut into rounds using a 2-inch cutter.

Bake for 10-12 minutes until edges are set, then transfer to a wire rack to cool completely.

Prepare the filling by beating 0.25 cup butter, powdered sugar, and heavy cream until smooth.

Spread a thin layer of buttercream and a small dollop of raspberry jam on the flat side of half the cookies.

Top with the remaining cookies to create sandwiches.