Preheat the oven to 325 degrees F and prepare a muffin/cupcake tin with cupcake liners.
Mix the flour, sugar, baking powder, salt and butter in an electric stand mixer fitted with the paddle attachment and beat on slow speed until you get a sandy consistency and everything is combined. Gradually pour in half the milk and beat until the milk is just incorporated.
In a separate bowl, whisk the egg, vanilla extract and remaining milk together for a few seconds, then pour into the flour mixture and continue beating until just incorporated (scrape any unmixed ingredients from the side of the bowl with a rubber spatula). Continue mixing for a couple more minutes until the mixture is smooth. Do not overmix.
Spoon the cake batter into your cupcake liners until about two-thirds full and bake for about 20-25 minutes, or until light golden and the cake bounces back when touched. (A skewer inserted in the center should come out clean.) Allow the cupcakes to cool slightly in the pan before turning out onto a wire rack to cool completely.
Once cooled completely, cut out the center of each cupcake (I used a strong, thick plastic straw). Fill your pastry bag with the marshmallow fluff and fill in the middle of each cupcake.