Place the egg whites in a bowl and leave out overnight. This step is necessary and cannot be skipped.
The next day, place the powdered sugar, almond flour, and raspberries in a food processor. Pulse about 10 - 15 times or until the raspberries are completely powdered and the whole mixture is pink.
Beat the egg whites in a completely cleaned and dried mixer until it gets frothy. Then add the salt and granulated sugar and beat until it forms stiff peaks.
Add the almond flour mixture and the food coloring, if using, to the egg whites.
Mix in the almond flour mixture until completely combined using a rubber spatula. This will take a bit of time, be patient and go slowly. You want to deflate the air from the egg whites, not add more in.
As soon as the almond flour is incorporated into the egg whites, carefully mix by folding the macaron mixture over onto itself. After every three folds, use your spatula to lift and slowly drop the mixture back into the bowl. If the mixture leaves the spatula in a ribbon, then it is ready. Do not mix anymore.
Leave the mixture uncovered in the bowl for about 30 minutes. While it is sitting, fit a piping bag with a large round tip.
Pour the macaron batter into the piping bag in batches, ensuring there are no bubbles in the bag.
Pipe the macarons into circle or hearts onto baking sheets lined with baking mats.
Tap the baking sheets onto a hard surface to release any air bubbles. Leave to rest for an hour.
Preheat the oven to 325 degrees F, and bake for 10 minutes, turning halfway through. Only bake one sheet of macarons at a time.
Remove the macarons and allow to cool.
While the macaron shells are cooling, make the ganache.
In a double boiler, add the chocolate, butter, heavy cream, and vanilla together. Heat until the chocolate and butter melts and stir to combine. Remove from heat and allow to cool completely.
While the ganache is cooling, match the macaron shells together that are of similar size.
Add the cooled ganache to one half of the macaron shell pairs, then top with the matching macaron shell.
Store in a cool, dry airtight container for up to several weeks.