To toast the marshmallows- You can either use a kitchen torch to toast until golden, toast on a skewer over a turned-on burner of your stove, or place the marshmallows on a lined sheet pan and put under the broiler of your oven for 2-3 minutes, or until toasted and puffed. Set the marshmallows aside to cool completely before starting the recipe.
Preheat your oven to 350 degrees and generously grease a 10-12 cup bundt pan. Set aside.
In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.
In a large bowl, beat together the softened butter, toasted marshmallows, and sugar until combined and fluffy. Beat in the vanilla extract. Add in the eggs, one at a time, mixing between each addition until combined.
Reserve about 1 tablespoon of the flour mixture. Add in the flour mixture and the whole milk in alternating additions, starting and ending with the flour mixture, and beat until the batter is smooth. Toss the reserved flour mixture with the chocolate chunks to coat, and fold the entire mixture (including any excess flour) into the batter until well distributed.
Pour the batter into your prepared pan, and spread evenly. Bake for 60-70 minutes, or until a cake tester comes out clean. If the cake begins to get fairly dark, gently cover in foil to stop browning.
Cool the cake for 30 minutes in the pan, and then turn out onto a cake stand or cooling rack to finish cooling. Slice and enjoy!