Make Chocolate Linzer Cookies - With a hand-held mixer or in the bowl of your stand mixer fitted with a paddle attachment, mix room temperature butter, powdered sugar and vanilla until light and fluffy. About 2 minutes on medium speed. 

Next, sift in all-purpose flour, unsweetened cocoa powder and salt. On the lowest setting, mix everything just until combined.

Divide the dough in half and transfer to plastic wrap and chill for at least 1 hour. I like to shape the dough into a rectangle, about 1/2 inch (1.27 cm) thickness to make it easier to roll out later.

Sprinkle cocoa powder or flour onto your work surface and roll your dough out to about a little over 1/8 inch (3.1 mm) thickness.

Lightly dip your cookie cutter into cocoa powder or flour and cut out your cookies. With a metal spatula, carefully transfer the cookies to a parchment lined baking tray, placing them 1/2 inch (1.27 cm) apart. Gather the scraps, reroll and cut out more cookies. I used a 2.5 inch (6.3 cm) cookie cutter and got 36 cookies. It may vary for you depending on how thick or thin you roll out your dough and the size of your cutter. See note below *

With a round 1-inch (2.5 cm) cutter, punch a hole in the center of 18 cookies, while leaving the other 18 cookies whole. Place the trays of cookies in the freezer for 20 minutes before baking. This is a very important step and cannot be skipped.

Bake at 350ºF (176ºC) for 10 to 12 minutes (depending on thickness). Allow to sit on the tray for several minutes, then transfer to a wire rack to cool completely.

Make Chocolate Ganache Filling - Finely chop your chocolate and transfer to a bowl. Add heavy cream to a small saucepan and heat until it reaches about 185ºF. Pour the hot cream over your chocolate, making sure to submerge it completely. Place a plate over the bowl and let stand for 3 minutes. Slowly mix until the ganache is incorporated. Allow to thicken at room temperature for 45 minutes.

Flip the "bottom" cookies over and pipe chocolate ganache onto the cookies, followed by the marshmallow fluff (measure with your heart). Place the top cookie on top of the filling and and lightly press down. Serve and Enjoy!!