• To make the cookies: pre-heat oven to 375°F. Line 2 large baking sheets with parchment paper or a silicone mat.
• In a medium bowl whisk together the flour, oat flour, baking powder, baking soda and salt. Set aside.
  2 cup flour, 1 cup oat flour, 1 teaspoon baking powder, 1 teaspoon salt, 1/2 teaspoon baking soda
• Using a small food processor or stick blender, puree the bananas with the lemon juice and vanilla extract.
  2/3 cup bananas, 1 teaspoon lemon juice, 1 teaspoon vanilla extract
• In the bowl of a stand mixer fitted with the paddle attachment cream together the butter and sugars until light and fluffy.
  3/4 cup unsalted butter, 1 1/4 cup toasted sugar
• Add the eggs, one at a time, beating well after each one until combined.
  2 egg
• Either using a silicone spatula to stir by hand or with the mixer on its lowest setting, alternate adding the dry ingredient mixture and the banana mixture. Start and end with the dry ingredients.
• Fold in the toffee pieces. The batter will be thick and creamy, closer to a very thick cake batter than cookie dough.
  1/3 cup toffee pieces
• Use a 1 1/2 tablespoon cookie scoop to drop uniformly sized portions of batter on the prepared baking sheet at least 2 inches apart. Bake for about 9-10 minutes; the cookies should be only lightly browned and springy to the touch.
• Transfer to a wire rack to cool completely.

To make the filling:

• In the bowl of a stand mixer, beat together the butter, cream cheese, meringue powder, heavy cream, salt and vanilla until creamy and smooth
  1 cup unsalted butter, 2 ounce cream cheese, 1 tablespoon meringue powder, 1 tablespoon heavy cream, 2 teaspoon vanilla extract, 1/2 teaspoon salt
• Beat in the marshmallow cream until well combined.
  5 ounce marshmallow cream
• Gradually beat in the powdered sugar, about 1 cup at a time until is is fully incorporated. The mixture should resemble a thick, dense whipped cream and hold stiff peaks. If it is too soupy, place the entire mixing bowl in the fridge for about 20 minutes and then whip again; repeat if necessary.
  3 cup powdered sugar

To assemble the cookies:

• Spread a layer of hot fudge on the backs of all the cookies, about 1 1/2 to 2 teaspoons per cookie.
  1 1/2 cup hot fudge
• Pipe a generous amount of the filling on the backs of half the cookies. Sprinkle the filling with toffee pieces.
  toffee pieces
• Place a second cookie on top of the filling and toffee pieces, pressing down gently until the filling nearly reaches the edges of the cookies.
• Store cookies in an air tight container.