Gently sift the powdered sugar and almond flour into a bowl to ensure they're perfectly fine and lump-free. Set this mixture aside.
In a very clean separate bowl, begin whisking your room-temperature egg whites with a hand or stand mixer. Start on low speed, then gradually increase to medium-high. Once they become frothy, slowly add the granulated sugar. Continue whisking until you achieve stiff, glossy peaks that hold their shape.
Carefully add the sifted almond flour and powdered sugar mixture to your whipped egg whites. If you'd like a pretty pink or red hue, add a few drops of food coloring now. Use a spatula to gently fold the ingredients together. Keep folding until the batter flows off your spatula like slow-moving lava but still holds its form – be careful not to overmix!
Transfer your beautiful batter into a piping bag fitted with a round tip. On a baking sheet lined with parchment paper, gently pipe small heart shapes by first drawing a "V" and then closing the bottom. Aim for similar sizes for even baking.
Allow your piped macaron hearts to sit at room temperature for 30 minutes to 1 hour. This important step helps them form a slight skin, which is key to getting that signature ruffled "foot" on your macarons.
Preheat your oven to 300°F (150°C). Once the shells have rested and developed their skin, bake them for 18 to 20 minutes. They should feel firm to the touch and easily lift off the parchment paper when perfectly baked.
While your macaron shells cool, beat the softened butter in a bowl until it's wonderfully smooth and creamy. Gradually add the powdered sugar, vanilla extract, and a tiny pinch of salt, mixing well after each addition. Stir in the heavy cream or milk, a tablespoon at a time, until your filling is smooth and easily spreadable. Add a few drops of red food coloring if you desire a colored filling.
Once the macaron shells are completely cool, gently flip them over. Pipe a small dollop of your delicious buttercream filling onto the flat side of one shell, then delicately top it with another shell to create a perfect sandwich.
For the absolute best flavor and chewy texture, place your assembled macarons in an airtight container and refrigerate them for at least 24 hours. This allows the filling to soften the shells and the flavors to beautifully meld together.