Preheat the oven to 350°F.
Grease a 9 x 2¼-inch heart-shaped baking pan and place two strips of parchment paper perpendicular to each other, allowing them to hang over the edges to act as pull handles.
In a medium bowl, whisk together the flour, cornstarch, salt, and baking soda. Set aside.
In a large mixing bowl, beat the butter and sugars on medium-high speed for 3-4 minutes until fluffy and light in color, scraping down the as needed.
Reduce the speed to low and add the eggs and vanilla extract. Mix until fully combined.
Add the flour mixture and mix until just combined. Do not overmix.
Fold in the chocolate chips.
Transfer ¾ of the cookie dough to the prepared pan. Smooth the layer with an offset spatula, making sure to work the dough into the corners of the pan.
Dollop the marshmallow cream onto the cookie dough layer and smooth it out with a lightly greased offset spatula.
Top with the remaining cookie dough, making sure that most of the marshmallow layer is covered.
Place the chocolate hearts on top and add sprinkles if desired.
Bake for approximately 25 minutes or until the edges turn golden brown, the center is just set, and a toothpick comes out mostly clean. The cookie cake will continue to cook as it cools.
Cool in the pan on a wire rack for 15 minutes. If needed, run a small offset spatula around the edges to release the cookie cake from the sides.
Use the parchment paper handles to lift the cookie from the pan and cool for another 10 minutes before slicing.