Creating Heart-Shaped Chocolate Sandwich Cookies is exciting and rewarding. Just follow these steps:

1. Preheat the Oven: Start by preheating your oven to 350°F (175°C). Line a baking sheet with parchment paper.

1. Mix Dry Ingredients: In a large mixing bowl, combine the flour, cocoa powder, baking soda, baking powder, and salt. Whisk until well mixed.

1. Cream Butter and Sugar: In another bowl, cream the softened butter and granulated sugar together until light and fluffy.

1. Add Egg and Vanilla: Beat in the egg and vanilla extract into the butter mixture until fully combined.

1. Combine Wet and Dry Ingredients: Gradually mix the dry ingredients into the wet ingredients, stirring until a dough forms. The dough will be soft but manageable.

1. Chill the Dough: Refrigerate the dough for about 15-20 minutes to make it easier to handle.

1. Roll and Cut Dough: On a lightly floured surface, roll out the chilled dough to about ¼ inch thickness. Use a heart-shaped cookie cutter to cut out shapes and place them on the prepared baking sheet.

1. Bake: Bake the cookies in the preheated oven for 10-12 minutes, until set. Allow them to cool on the baking sheet for 5 minutes before transferring them to wire racks to cool completely.

1. Prepare Filling: In a bowl, combine the powdered sugar, cocoa powder, and milk to create a smooth filling. Adjust the milk quantity to achieve your desired consistency.

1. Assemble the Cookies: Once the cookies are completely cool, spread a dollop of filling on the flat side of one cookie and place another cookie on top, pressing down gently. Repeat this for all cookies.

1. Decorate: If desired, dust the finished sandwiches with a little powdered sugar or drizzle melted chocolate over them for a decorative touch.