Preheat the oven to 350 (F (180 (C) and line 2 baking trays with parchment paper.
Sift the flour, cocoa powder and baking powder into a large bowl. Stir in the brown sugar. Add the butter and mix in using electric beaters on medium speed (or in a mixer) until pieces of butter are no longer visible (the mixture will look like cookie crumbs).
In a separate bowl, whisk the milk, egg and vanilla. Add this all at once to the dry ingredients and mix on low speed to incorporate. Increase the speed to medium high, beating for about 2 minutes until the batter is light and fluffy.
Using an ice cream scoop (a 3/4-oz/ 22 mL) scoop will yield 40 pieces) scoop the batter onto the prepared baking trays, leaving 2 inches (5 cm) between each scoop.
Bake for 13-15 minutes, until the top of the "cake" springs back when gently pressed. Cool the "cakes" completely on the baking tray before removing with a spatula.
For The Filling:
Place the milk chocolate chips in a metal bowl placed over a pot filled with an inch of gently simmering water, stirring with a spatula until melted (alternatively, you can melt the chocolate chips in the microwave on medium heat, stirring every 10 seconds until smooth) Set aside to cool slightly.
Beat the cream cheese with electric beaters (or in a mixer) until smooth, and then add half of the icing sugar, beating again until smooth (the filling will be runny). Add the melted milk chocolate, beating to incorporate, and then add the remaining icing sugar, beating on high speed until fluffy.
To fill, spread a generous amount (3-4 tbsp/45-60 mL) onto to the bottom of one "cake" and then sandwich the filling with a second "cake". Alternatively, you can pipe the filling using a piping bag fitted with a large star or plain tip. The whoopie pies can be stored chilled for up to 3 days, but are best enjoyed pulled from the fridge 30 minutes before serving.
More information and baking inspiration is available at www.hersheyskitchens.ca.