Preheat the oven to 300°F. Line two baking sheets with silicon mats that have macaron cookie outlines, or with parchment paper. Set aside.
In the bowl of the stand mixer with the whisk attachment, beat the egg whites until foamy, about one minute. Add in the meringue powder.
Slowly add granulated sugar about one-third at a time, mixing well at medium speed after every addition. Once all the sugar has been combined, increase the speed to medium-high and mix for another five minutes. Mixture should form very stiff peaks and be balled up in the whisk attachment, pulling away from the sides of the bowl.
In a fine mesh sieve over a large bowl, sift the almond flour, powdered sugar, and egg white powder. If there are any large chunks left that do not sift easily, discard them.
Pour the sifted dry ingredients into the whipped mixture and mix on low, just until it is incorporated.
Using a silicone spatula, gently fold the batter, scraping the bottoms and sides of the bowl as you fold, to ensure there are no dry ingredients left unmixed. To check the consistency, pull some of the mixture up with your spatula and let the mixture drop back into the bowl. When it’s ready, it should flow nicely off the spatula, ribboning as it falls and then re-incorporating back into the mixture within about a 20 second count. Keep gently folding just until the mixture reaches this level.
Transfer the batter to a pastry bag fitted with a large round metal tip. If you do not have a tip, then use scissors to cut the end of the pastry bag in a small straight line.
Hold the bag approximately ½ inch above the prepared baking sheet and perpendicular to the pan, using the center as a bullseye. Slowly squeeze the bag, keeping it in one place until the mixture reaches the inner circle of the template. The mixture should settle to the outer line. Repeat, piping out all the rounds until you've used all of the batter.
Firmly bang the baking sheet on the counter several times. This helps release any air bubbles and helps the batter settle. If you see any stubborn air bubbles remaining, you can pop them carefully with a toothpick.
Rest the piped rounds to rest for at least 30 minutes. If you live in a high humidity climate, increase this rest time. When ready to bake, the tops should no longer be sticky. If you think the shells are ready to bake, you can test one shell by gently touching the surface. If it sticks to your finger, it’s not ready yet. Allow it to rest until you can touch it without sticking to your finger.
Once they have rested for about 30 minutes and are no longer sticky, bake them for 12 minutes. It's best to bake one baking sheet at a time. If baking more than one sheet at a time, make sure to rotate the pans halfway through the cooking time. NOTE: this is a good time to clean out the stand mixer bowl and whisk attachment, since you will need them again to make the filling.
Once the macaron shells are fully baked, allow them to cool on the pan. Once cooled, very gently remove them from the baking sheet.
Store the macaron shells in an airtight container stacked on their sides until you're ready to add the filling.
Make the tiramisu filling
In a large microwave safe bowl, add the white chocolate chips, heavy cream, and Kahlua. Microwave for 30 seconds at a time and stir well between each interval until the mixture is smooth.
Transfer the filling to the bowl of your stand mixer and allow to cool. Once cool to the touch, use the whisk attachment to whip the ganache on medium speed for about a minute or two.
Add in the mascarpone cheese and the powdered sugar. Mix just until combined; it's important to not overmix in this step.
ASSEMBLE
When ready to assemble, transfer the filling to a pastry bag fitted with a large round tip and pipe a dollop of filling on a shell. Then place another shell on top of the filling to create the cookie sandwich. Repeat until you've filled and topped all shells.
Dust the macarons with cocoa powder just before serving.
Notes
Makes approximately 25-27 macarons.While you can enjoy macarons right away, it’s best to wait 24 hours for the shells to mature and create that classic macaron chewy goodness.Storage: Store in an airtight container in the refrigerator for about two weeks. You may also freeze macarons for up to three months. If stored in the freezer, take the macarons out of the freezer and allow them to come to room temperature before enjoying.