Prepare macaron shells as directed and pair up by size. Set aside.
In a small saucepan, bring the champagne to a simmer over medium heat. Allow to simmer about 10 minutes, until reduced to ¼ cup. Remove from heat and let cool completely.
To make the frosting, combine the egg whites, sugar and salt in a heatproof bowl set over a pot of simmering water. Heat, whisking frequently, until the mixture reaches 160° F and the sugar has dissolved.
Transfer the mixture to the bowl of a stand mixer fitted with the whisk attachment. Beat on medium-high speed until stiff peaks form and the mixture has cooled to room temperature, about 8 minutes. Reduce the speed to medium and add the butter, 2 tablespoons at a time, adding more once each addition has been incorporated. If the frosting looks soupy or curdled, continue to beat on medium-high speed until thick and smooth again, about 3-5 minutes more (don’t worry, it will come together!)
Add the champagne 1 tablespoon at a time, allowing each addition to be fully incorporated before adding the next. It may seem to separate, but the frosting will come back together after another minute or two of beating. Transfer to a pastry bag fitted with a plain round tip. Pipe a small dollop of the champagne frosting onto the flat side of one cookie of each pair. Sandwich together with the remaining cookie, pushing the filling to the edges. Top with a sprinkle of edible glitter, if desired. Store in an airtight container.