In a large mixing bowl, cream together the butter, light brown sugar, and cane sugar until light and fluffy.
Add the milk and vanilla extract, and mix until smooth and fully incorporated.
In a separate bowl, whisk together the flour, cocoa powder, cornstarch, baking soda, baking powder, and salt.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined to form a soft cookie dough.
Cover the dough and let it rest at room temperature for 30 minutes.
Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
Use a 2-tablespoon (50g) cookie scoop to portion out the dough. Flatten each portion into a disc in your hand as shown in the photos.
Place a frozen ganache scoop in the center of each dough disc, then carefully wrap the dough around the ganache, sealing the edges to completely enclose the filling.
I have rolled some dough balls in cane sugar, festive sprinkles, or cocoa powder for an assortment of cookies.
Space the dough balls at least 3 inches apart on the prepared baking sheet.
Bake the cookies in the preheated oven for 10–12 minutes. The edges should be set while the centers remain slightly soft.
Immediately after baking, use a round cookie cutter slightly larger than the cookie to gently swirl around the edges. This helps create perfectly rounded edges and a more uniform shape.
Let the cookies cool on the baking sheet for 5–10 minutes. If you want a gooey molten center, serve them warm or cool them completely for gifting in your festive hampers.