Grease a 9x13x2-inch pan with butter or baking spray and preheat the oven to 350°F.
In the bowl of a stand mixer fitted with the whisk attachment, combine the flour, sugar, baking powder, baking soda and kosher salt. Add the butter and run on medium-low speed until the butter is well incorporated and the mixture looks sandy. There should be no lumps of butter. (You could also do this with a hand mixer it will just take longer.)
In a large glass measuring cup or small mixing bowl, combine the buttermilk, vegetable oil, vanilla and eggs.
Switch the mixer to the paddle attachment. With the mixer on running on low, gently stream in the liquid ingredients. Once all the liquid has been added, scrape the bottom and sides of the bowl and return to mix on low for a few more seconds.
Pour the batter into the prepared tin and spread it out evenly with a rubber spatula, offset spatula or the back of a spoon.
Scatter the chocolate chips evenly over top. You should still be able to see plenty of batter underneath.
Place on the center rack and slide a half sheet tray on a rack underneath to catch any marshmallow spillage. Bake for 28 minutes and then very gently pull out the oven rack the cake is on and top the cake with the mini marshmallows, just enough to barely cover the surface. This doesn’t have to be perfect (they will melt and spread out anyway) but you do want to get it back in the oven very quickly to prevent it from deflating any further.
Bake for 7 more minutes. A wooden skewer poked into the center of the cake should come out with very moist crumbs.
Transfer to a wire rack to let cool completely before serving.
