Combine the flour, brown sugar, baking soda and salt in the bowl of a food processor fitted with the steel blade or in the bowl of an electric mixer fitted with the paddle attachment. Pulse or mix on low to incorporate. Add the margarine and pulse on and off, on and off, or mix on low until the mixture is the consistency of a coarse meal.
In a small bowl, whisk together the honey, milk and vanilla extract. Add to the flour mixture and pulse on and off a few times or mix on low until the dough barely comes together. It will be very soft and sticky. Lay out a large piece of plastic wrap and dust it lightly with flour, then turn the dough out onto it and pat it into a rectangle about 1 inch thick. Chill until firm, about 2 hours or overnight.
Preheat oven to 350. Remove half of the dough from the refrigerator and roll it out between two pieces of parchment paper. Cut out circles using a round cookie cutter or the top of a drinking glass and transfer the circles to a parchment lined cookie sheet. Fill four of the circles with a small amount of fluff (less than a teaspoon) and sprinkle with a small amount of chocolate. Use significantly less filling than might seem right because marshmallow fluff expands in the oven. Fold up 2 sides of the circle and pinch them together. Lift the third side and pinch the remaining two corners closed. Repeat with the remaining circles. (I say to only fill 4 hamentashen at a time because fluff begins to ooze and you need to work fast to fold the dough before the filling has oozed too much.) Roll out and fill remaining dough.
Bake hamentashen for 8-10 minutes or until set. Transfer to a cooling rack and let cool completely before enjoying.