Preheat the oven to 350°F. Line two baking sheets with parchment paper.
In a medium bowl, whisk together your flour, cocoa powder, baking soda, and salt.
In your large mixing bowl, beat the softened butter until it’s creamy – about 30 seconds. Add both sugars and beat until the mixture looks fluffy and lightens in color, about 2-3 minutes.
Beat in eggs one at a time, then vanilla, scraping down the sides of the bowl as needed. The mixture might look a bit curdled – that’s totally normal.
Gradually stir in the flour mixture until just combined.
Drop rounded tablespoons of dough onto your prepared sheets, leaving about 2 inches between each. Make a small thumbprint in the center of each cookie to hold the marshmallow creme.
Spoon about 1 teaspoon of marshmallow creme into each indent. Using the tip of a knife or toothpick, swirl it gently into the surrounding dough.
Bake for 11-13 minutes, rotating sheets halfway through. You’ll know they’re done when the edges are set but the centers still look slightly soft – they’ll continue cooking a bit on the hot baking sheet.
Let them cool on the baking sheets for 5 minutes – they’re quite fragile when hot. Then transfer to a wire rack to cool completely. The marshmallow swirl will settle as they cool.