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

In a bowl, sift together powdered sugar and almond flour.

In a large bowl, beat egg whites and cream of tartar until soft peaks form. Gradually add granulated sugar and beat until stiff peaks form. Beat in vanilla extract and food coloring if using.

Fold the almond flour mixture into the egg white mixture until just combined.

Transfer the batter to a piping bag fitted with a round tip. Pipe 1-inch circles onto the prepared baking sheets, spacing them 1 inch apart.

Tap the baking sheets firmly on the counter to release any air bubbles. Let the macarons sit at room temperature for 30 minutes to 1 hour, until they form a skin and are no longer sticky to the touch.

Bake for 18-20 minutes, until set but not browned. Let cool completely on the baking sheets.

For the filling, beat together butter, powdered sugar, vanilla extract, and heavy cream until smooth. Add food coloring if desired. Transfer to a piping bag and pipe onto half of the macarons. Top with the remaining macarons to form sandwiches.