Bring egg whites to room temperature in the bowl of a stand mixer with a whisk attachment (make sure the bowl is COMPLETELY clean - no oil residue or water!). Meanwhile, combine the dry ingredients (almond flour, powdered sugar, and strawberry powder) in a food processor and pulse several times until combined. This step isn't totally necessary, but it helps even out lumps from the ingredients and makes the macarons finer. Then, sift at least once (I like to do it twice), discarding any lumps and/or seed bits from the strawberries. Set aside.

Prepare piping bag by placing a large, round piping tip inside (I use Wilton's 2A) and folding the bag into the tip to "close" it - this helps prevent the batter from seeping out before you pipe it. Place in a tall upright glass to hold while you pour in the batter. Also, prepare the large baking sheet with the Silpat or parchment paper. Set aside but nearby because you'll need them immediately after the batter is fully mixed!

Once the egg whites are at room temperature, turn the mixer on medium - around setting 4. Once they are foaming (~3 minutes in), add the cream of tartar. You don't have to add it, but it helps to stabilize the meringue. Continue beating, increasing the speed to around 6. Once the moisture has left the egg whites and the whisk begins leaving "tracks", slowly add the sugar, about a tablespoon at a time. Continue beating for approximately 6-10 more minutes. You want the meringue to have stiff peaks, so if the bowl is turned upside down it doesn't move. When the beater is pulled up, it should have a straight peak, not one that falls or curves.

Immediately after the meringue is ready, add about 1/3 of the dry ingredient mixture. Begin folding it in, pulling up from the bottom and gently folding in. This part is going to take some muscle work, as your arms will get tired from all the folding - hang in there! Once the dry ingredients are incorporated, add about 1/3 more of the remaining dry ingredients. Continue folding in until combined, and add the remaining dry ingredients. You will continue to fold the batter (and pushing some against the side as needed to release some air) until it "flows like lava". I loved using the "figure 8" test - the batter should be able to flow off of the spatula long enough for you to write a figure 8. The batter should also "melt" back into itself after about 30 seconds if you create ribbons with it. Be careful not to overmix (this does take practice!). 

Immediately place the batter in the prepared piping bag. Squeeze the batter gently into the tip of the bag, and keeping the bag completely verticle, pipe approximately 1 1/4 inch circles onto your Silpat or parchment (both of which can have templates). 

Bang the pan on your counter several times (yes, BANG it) to remove the air bubbles. Sometimes I even take a toothpick and gently pop some that just won't budge. Let the macarons dry for 30 minutes to an hour (this will vary depending on your environment!). The should have a matte appearance and you will be able to run your finger over the top without any batter sticking). Approximately 15 minutes into the dry time, turn the oven to 290°F. This may vary with your oven! You may need to adjust as you see fit once you begin baking the macarons. It will probably require some testing. I went between 290-305°F. 

Once the macarons are dry, place in the oven for 16-20 minutes (if you adjusted the temperature, this will vary, again a thermometer is super helpful as your oven is most likely NOT accurate!). The shells should not move when gently tapped. Remove and place the pan on a cooling rack. 

As the shells cool, make the buttercream filling. Cream butter for about 2 minutes until smooth using an electric mixer. Add remaining ingredients and mix on low until combined, then increase the speed to high for approximately 3 minutes until smooth. Set aside. 

Once macarons are cooled, pick up one corner of the Silpat/parchment to gently pull them away (kind of like removing a sticker). Match pairs up by size. Using the same tip, pipe a dollop of buttercream into the center of one of the pairs - you'll want to leave an "edge" so when you sandwich them together, the filling doesn't ooze out. Place the other half on top and push gently together until they are sandwiched. Continue with the remaining cookies.

Store in an airtight container in the fridge. Macarons have their best flavor when stored for 24-48 hours and then brought to room temperature again.