Pamplemousse curd with cardamom:
In a bowl, mix the grapefruit juice with the sugar.
Add the beaten eggs, cornstarch, butter, and cardamom.
Place the bowl over a double boiler.
Whisk until the mixture thickens and has the texture of a pastry cream.
Let cool to room temperature before refrigerating.

The shells:
In a food processor, mix the almond powder and powdered sugar until you get a very fine powder.
Sift the resulting powder and set aside.

Warm the egg whites for 10 seconds in the microwave at low power.
Beat them into a snow with the cream of tartar.
When the whisk starts to leave marks, add the sugar and Poplar Baumier extract while whisking at high speed.

Add a few drops of coloring (the amount depends on the desired color; start with a small amount and adjust to taste; keep in mind that the macaron will be a bit lighter once cooked).

Give it another whisk to incorporate the coloring.

Gradually and gently incorporate the sifted powder with a silicone spatula, being careful not to break the egg whites.
The mixture should be smooth and shiny.

Fill a piping bag.
To do this, twist and push the bottom of the bag into the hole of the tip, place the bag in a pitcher to hold it, and pour the batter into the bag.
Close the bag.

Pipe the macarons onto two baking sheets lined with silicone mats (I used the red mats sold at Canadian Tire), spacing them about 1 1/4 inches (3 cm) apart.
The macarons should normally spread out.
If there is still a point on top, it means the batter is too thick.
Let them sit for at least 1 hour (in a relatively dry atmosphere).

Preheat the oven to 285°F (140°C).
Bake the macaron sheets.
Bake for 15-20 minutes.
After 5-6 minutes, the collar should start to form.

Remove the sheets from the oven and let them rest for 30 seconds to 1 minute, then remove the macarons, which should come off easily if they are well cooked underneath.
Wait a bit if they resist coming off.
Place them on their side so that the base can dry.

Match the macarons in pairs according to their size and assemble them by gluing them with the filling.

Once the macarons are filled, refrigerate them overnight (flat, so the filling can set and the macarons can “set” well).

Wait at least one day before serving.
The macarons are best after 2-3 days.
They can be stored in the refrigerator for a good week, but you can also freeze them (to thaw, leave them at room temperature for at least an hour).