In the bowl of a food processor, add 70 gm almond flour and 120 gm powdered sugar. Pulse twice then process 1 1/2 min. Sift and discard any solids (should be less than 1 Tbsp solids in sieve).
Put 2 egg whites in large clean/dry mixing bowl and beat until foamy, then gradually add 1/4 cup granulated sugar and beat on high speed about 2 1/2 minutes or until stiff glossy peaks form. Add 1" strip red gel food coloring and fold with spatula just until incorporated.
Add all sifted almond flour/sugar mixture and fold with spatula using this method: scrape batter around the bowl, then press the flat side of the spatula firmly through the center. Repeat until batter flows like lava and thick ribbon forms as it flows off spatula (35-40 complete strokes).
Print 4 copies of heart template and place on 2 baking sheets under silicone liners or parchment paper (AVOID WAX PAPER) - you'll be able to see the hearts through your liner to guide you as you pipe hearts. Transfer batter to large piping bag fitted with 3/8" round tip. For each heart, pipe at the top left and pull down towards the bottom center then pipe at the top right and pull down towards the bottom center and flick the end up if it gets long.
Add sprinkles to as desired. Firmly tap baking sheets over counter 15-20 times to release air bubbles. Poke any remaining obvious bubbles with toothpick. Let cookies sit at room temp 30 min or until no longer sticky to the touch - they should form a thin film on top. Meanwhile preheat oven to 300˚F.
Bake one sheet at a time for 12-15 min at 300˚F. I usually do 15 minutes for the first batch and 12 minutes for the second batch - somehow my oven gets hotter the second batch. Let cookies cool to room temp before removing them from baking sheet. Once cool, they should slide off easily. Meanwhile make buttercream (see instructions below).
To assemble, flip over half your cookies and pipe generous amounts of frosting onto the flat sides, piping in a heart shape. Find a well-matched unfrosted second half and sandwich cookies together.
How to Make Lemon Buttercream:
In a medium mixing bowl, cream 6 Tbsp softened butter on high speed for 2 mins.
Add 1 1/2 cups powdered sugar (1/2 cup at a time and beating to incorporate between additions), along with 1/8 tsp of salt and once sugar is incorporated, increase to high speed and beat 2 mins.
Add 2 Tbsp lemon juice and beat another 1 minute or until whipped and fluffy. Transfer to the same clean piping bag fitted with the round 3/8" tip.