Preheat your oven to 300°F (150°C) and line two baking sheets with parchment paper.

In a bowl, sift together the powdered sugar, almond flour, and salt. Set aside.

In a separate bowl, beat the egg whites and cream of tartar with an electric mixer on medium speed until foamy. Gradually add the granulated sugar and continue to beat until stiff peaks form. This should take about 5 minutes.

Gently fold in the vanilla extract and a few drops of red or blue gel food coloring until the desired color is achieved.

Carefully fold the almond flour mixture into the egg whites in three additions, being careful not to deflate the meringue. The mixture should flow like lava when ready.

Transfer the macaron batter to a piping bag fitted with a round tip. Pipe 1.5-inch circles onto the prepared baking sheets, leaving space between each.

Tap the baking sheets on the counter to release any air bubbles. Let the macarons sit at room temperature for 30-60 minutes, or until they form a skin and are no longer sticky to the touch.

Bake in the preheated oven for 15-18 minutes, rotating the sheets halfway through. The macarons should have risen and formed a 'foot.'

Allow the macarons to cool completely on the baking sheets before removing them.

For the filling, beat the softened butter until creamy. Gradually add the powdered sugar, vanilla extract, and heavy cream until smooth and fluffy.

Pipe the filling onto the flat side of half the macarons and sandwich with the remaining halves.