Add the butter, granulated sugar and brown sugar to a large bowl using a hand-held or stand mixer fitted with a paddle attachment. Beat on medium-high speed for 2-3 minutes, until it's light and fluffy in color. With the mixer still on, beat in the egg, vanilla extract, and milk, scraping down the sides and bottom of the bowl as needed.
1/2 cup (113g) unsalted butter, 1/2 cup (100g) granulated sugar, 1/2 cup (100g) light brown sugar, 1 large egg, 1 teaspoon pure vanilla extract, 1 Tablespoon (15ml) milk
In a separate medium bowl, whisk the flour, cocoa powder, baking soda and salt together until combined. Turn the mixer down to low and slowly add the dry ingredients into the wet ingredients. Continue mixing until combined.
1 cup (125g) all-purpose flour, 2/3 cup (55g) natural cocoa powder, 1 teaspoon baking soda, 1/2 teaspoon kosher salt
Add the chocolate chunks and mix using a rubber spatula to incorporate. Cover the dough tightly with plastic wrap and chill for at least 3 hours and up to 3 days. Do not skip this step.
1 (4 ounce) bar chopped semi sweet chocolate bar
Preheat the oven to 350°F and line two large baking sheets with parchment paper.
Remove the cookie dough from the refrigerator. If it's been chilling for more than 24 hours it may need to sit at room temperature for 20-30 minutes so it's easier to handle. Dollop the marshmallow fluff on top and use a rubber spatula to gently fold it into the cookie dough, leaving large streaks of marshmallow throughout.
1/2 cup marshmallow creme (fluff)
Use a large cookie scoop to drop the cookie dough onto the prepared baking pans a few inches apart. Roll the dough into balls that are more tall than they are round, like a cone (see blog post image).
Bake the cookies in the preheated oven for 10-12 minutes or until the edges are set and the tops look slightly soft. If the cookies aren't perfectly circular, place a large cup upside down over top and gently swirl it around while they're still warm to reshape the edges. Add some extra chocolate chunks to the top of the cookies. Transfer to a wire rack to cool completely.