For the Macaron Shells:

1. Preheat the oven to 300°F (150°C). Line two baking sheets with parchment paper or silicone baking mats.
2. In a food processor, sift together the powdered sugar and almond flour to remove any lumps.
3. In a clean, dry bowl, beat the egg whites using an electric mixer on medium speed until soft peaks form. Gradually add the granulated sugar and continue beating on high speed until stiff, glossy peaks form.
4. Add the vanilla extract (and food coloring, if desired) and gently fold to combine.
5. Carefully fold the dry ingredients into the egg whites, a little at a time, using a spatula. The batter should flow like lava and form ribbons when lifted with the spatula. Be gentle to avoid deflating the meringue.
6. Transfer the batter to a piping bag fitted with a round tip. Pipe heart shapes onto the prepared baking sheets, leaving about 1 inch of space between each macaron.
7. Tap the baking sheet firmly on the counter to release any air bubbles, then let the macarons sit at room temperature for about 30 minutes, or until they form a skin and are no longer tacky to the touch.
8. Bake the macarons in the preheated oven for 15-18 minutes, or until they easily lift off the parchment paper. Let them cool completely on the baking sheets.

For the Buttercream Filling:

1. Beat the softened butter in a mixing bowl until smooth and creamy.
2. Gradually add the powdered sugar, ½ cup at a time, and beat until fluffy.
3. Add the milk, vanilla extract, and red food coloring (if using), and continue mixing until smooth and creamy.

Assembling the Macarons:

1. Once the macaron shells are completely cool, pair them up by size.
2. Pipe a small dollop of buttercream filling onto the flat side of one macaron shell.
3. Gently press the second macaron shell onto the filling, sandwiching them together.
4. If desired, refrigerate the macarons for 24 hours to allow the flavors to meld and the texture to soften.
5. Allow the macarons to come to room temperature before serving.