Prepare the dough:
Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper.
In a large bowl, cream together the softened butter, brown sugar, and granulated sugar until smooth and fluffy, about 2-3 minutes.
Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
In another bowl, whisk together the flour, cocoa powder, baking soda, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Stir in the semisweet chocolate chips, milk chocolate chips, and dark chocolate chunks (if using).

Shape and bake the cookies:
Using a tablespoon or cookie scoop, drop dough onto the prepared baking sheets, spacing the cookies about 2 inches apart.
Bake for 10-12 minutes, or until the cookies are set but still slightly soft in the center. They will firm up as they cool, so be careful not to overbake them.
Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.

Make the fudgy filling (optional):
While the cookies are baking, you can make the fudgy filling for an even more decadent treat.
In a small saucepan, heat the heavy cream over medium heat until it just begins to simmer (do not boil).
Remove from heat and pour over the semisweet chocolate chips. Stir until smooth and glossy. Let it cool slightly to thicken.

Assemble (optional):
Once the cookies are completely cooled, spoon a small amount of the fudgy filling onto the center of one cookie. Top it with another cookie to create a sandwich.
For an extra indulgent touch, drizzle some of the remaining fudgy filling on top of the cookie sandwiches.

Serve and enjoy:
Serve the cookies as they are, or go all out with the fudgy filling. Enjoy these decadent, gooey, chocolate-packed cookies with a glass of milk or your favorite beverage!