Age egg whites by separating them and leaving them in a covered container in the fridge for 24 hours; bring to room temperature before whipping.

Preheat the oven to 300°F (150°C) with racks in the middle and lower positions. Note: Do not count preheat as Step 1.

Line trays with silicone mats or parchment and place your heart template under the sheet if desired.

Sift almond flour and powdered sugar together twice into a large bowl. Discard or re-grind any larger almond bits.

In a clean bowl, whip the egg whites with cream of tartar until foamy.

Slowly add granulated sugar while whipping to medium-stiff peaks. Add gel color near the end and whip until glossy peaks form.

Fold one third of the dry mix into the meringue with a spatula to loosen it, then add the rest in two additions.

Test the batter by lifting the spatula: the ribbon should fall slowly and blend back into the batter in about 10 seconds.

Fill a piping bag with the batter and pipe hearts using a template under your mat or parchment.

Hold the bag vertical and squeeze with steady pressure.

Tap the tray gently to remove air bubbles, then use a toothpick to pop any remaining bubbles.

Rest the piped shells at room temperature for 30-60 minutes or until they form a dry skin on top.

Bake at 300°F (150°C) for 12-15 minutes. Rotate trays halfway through if your oven has hot spots.

Cool the shells completely on the tray before removing to prevent tearing.

For buttercream: Beat softened butter until smooth, then gradually add powdered sugar, cream, and salt.

For ganache: Heat cream until it simmers, pour over chopped dark chocolate, let sit for 2 minutes, then whisk until smooth. Chill to a spreadable consistency.

Match shells by size, pipe filling on the flat side, and sandwich. Press gently to spread filling to the edge.