Heat oven to 375 degrees.
Coat a baking sheet with cooking spray.
For the cakes: Mix the flour, baking powder, baking soda and salt in bowl.
In another bowl, beat 1 cup of butter and 1 cup of packed brown sugar until creamy.
Add vanilla and eggs to the butter mixture and beat 30 seconds.
Add the cocoa and beat to combine.
Next, beat in the flour mixture and ¾ cup of milk, alternating the two, a little at a time until the batter is smooth.
Drop mixture onto prepared sheets in two-tablespoon mounds, spacing 3 inches apart. If needed, shape each into equal-sized rounds.
Bake for 12 to 14 minutes or until cookies spring back when lightly touched in center.
Cool on sheets 5 minutes then remove from cookie sheets and place on a rack to cool completely.
For filling: Beat confectioner's sugar and butter slowly until blended.
Add vanilla and up to 1 cup of milk, ¼ cup at a time, mixing after each addition. You want the frosting to be blended and spreadable, but not runny.
Spread frosting onto the flat side of one cookie.
Place a second, similar-sized cookie on top, pressing lightly until filling begins to show around edges.
Store in an airtight container, layered between sheets of waxed paper, up to 3 days in the fridge or in the freezer up to a month.