In a large mixing bowl, beat the butter, granulated sugar, and brown sugar until light and fluffy, about 2–3 minutes.
Add the eggs and vanilla extract. Beat until combined.
In a separate bowl, whisk together the flour, cocoa powder, baking soda, and salt.
Gradually mix the dry ingredients into the butter mixture until a soft chocolate cookie dough forms.
Cover and refrigerate the dough for 30 minutes.
Preheat the oven to 350°F. Line two baking sheets with parchment paper.
Scoop the dough into 24 equal portions, approximately 1 1/2 tablespoons each. Roll into balls and gently flatten into thick disks about 2 inches wide.
Arrange the cookies about 2 inches apart on the baking sheets. Bake for 9–11 minutes, until the edges are set and the centers still look slightly soft.
Let the cookies cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely.
To prepare the buttercream, beat the softened butter until creamy. Gradually add the powdered sugar, vanilla, salt, and enough milk or cream to create a thick, smooth, pipeable frosting.
Divide the buttercream among three bowls. Tint each portion orange, purple, or green using gel food coloring.
Pair the cooled cookies by size. Pipe a generous ring of colored buttercream onto the flat side of one cookie in each pair, keeping the filling slightly inside the edge.
Place a second cookie on top and press gently until the buttercream becomes visible around the edges.
Press candy eyeballs into the exposed buttercream around the front of each sandwich. Roll or sprinkle the remaining exposed buttercream edges with Halloween sprinkles.
Refrigerate for 10–15 minutes if needed to firm up the filling before serving.