Position oven racks in the centre and bottom thirds of oven. Preheat to 250F. Line 2 baking sheets with parchment. Have a large pastry bag fitted with a No. 5 round tip ready.
Pulse cashews in a food processor until finely ground. Set aside.
Combine egg whites with cream of tartar and vanilla in a large bowl. Beat, using an electric mixer on medium, until soft peaks form when beaters are lifted, 1 to 2 min. Beat in food colouring, then granulated sugar, 1 tbsp at a time. Increase speed to medium-high and beat until whites are shiny and firm peaks form when beaters are lifted, 2 to 3 more min. Stir flour with 1/2 cup ground cashews in a small bowl, then fold into meringue mixture. Fill pastry bag with meringue mixture. Pipe 1 1/2-in. rounds onto prepared baking sheets, about 1/2 in. apart. Meringues will spread very slightly as they bake.
Bake in centre and bottom thirds of oven, switching racks halfway through, until firm but not golden, 40 to 50 min. Let stand on baking sheets 10 min, then transfer to a rack to cool completely.
Beat butter with icing sugar and liqueur in a medium bowl until pale and smooth. Turn all the meringues over and lightly spread buttercream (about 1/4 tsp) on the flat side of each cookie. Spread remaining ground cashews in a thin, even layer on a plate. Dip half of the meringues in cashews and sandwich each with another cookie. Store in an airtight container and refrigerate up to 2 days or freeze up to 1 month. (Prep Tip: No pastry bag? Fill a zip-top freezer bag with meringue mixture. Snip off a corner measuring 1/2 in. wide.)