1. Line two baking sheets with parchment paper and preheat the oven to 350°F (175°C).

2. In a large mixing bowl, beat the softened butter, granulated sugar, and brown sugar together until light and fluffy, about 2 to 3 minutes.

3. Add the eggs one at a time, mixing well after each addition, then beat in the vanilla extract until smooth.

4. In a separate bowl, whisk together the flour, cocoa powder, baking soda, baking powder, and salt until well combined.

5. Add the dry ingredients to the wet ingredients in two or three additions, mixing on low speed just until no streaks of flour remain.

6. If the dough seems very stiff or crumbly, add the milk or cream and mix briefly until the dough is soft and scoopable.

7. Use a small cookie scoop or tablespoon to portion the dough into 1 to 1 1/2 tablespoon balls and place them on the prepared baking sheets, leaving about 2 inches between each cookie.

8. Bake the cookies for 8 to 10 minutes, until the edges look set but the centers are still slightly soft and puffy.

9. Let the cookies cool on the baking sheets for 5 minutes, then transfer them to a wire rack to cool completely before filling.

10. While the cookies cool, make the chocolate filling by placing the chocolate chips in a heatproof bowl and heating the cream in a small saucepan just until steaming.

11. Pour the hot cream over the chocolate, let sit for 1 minute, then stir until completely smooth and glossy; add the butter and stir until melted and combined.

12. Allow the chocolate filling to cool at room temperature, stirring occasionally, until it thickens to a spreadable consistency.

13. To make the vanilla cream filling, beat the softened butter in a medium bowl until creamy and smooth.

14. Gradually add the powdered sugar, mixing on low speed at first, then increasing to medium until the mixture is thick and smooth.

15. Add 2 tablespoons of cream or milk, the vanilla extract, and salt, then beat until the filling is light and fluffy; add the remaining cream if needed to reach a soft, spreadable texture.

16. Pair the cooled cookies by size and place them flat-side up on your work surface.

17. Spread or pipe a layer of chocolate filling onto the flat side of half of the cookies.

18. Pipe or spread a layer of vanilla cream filling on top of the chocolate layer, leaving a small border around the edge.

19. Top each filled cookie with another cookie, flat side down, and gently press until the filling spreads just to the edges.

20. Let the sandwich cookies rest at room temperature for 20 to 30 minutes so the fillings can set before serving or storing.