How to Make It
In a large bowl beat the butter and ½ cup sugar on medium speed until fluffy. Reduce the speed and gradually add the flour, mixing just until combined. Add the egg yolk and mix once more to incorporate. Shape the dough into a disc, wrap with plastic wrap, and chill for 1 hour.
Make the meringue: With an electric mixer, beat the egg whites in a large bowl until foamy. Increase the speed to high and gradually add the remaining ½ cup sugar, 1 tablespoon at a time. Continue to beat, scraping down the sides of the bowl as necessary, until stiff peaks form, 5 to 8 minutes.
Line two baking sheets with parchment paper. Divide the chilled dough in half. Working on a lightly floured surface, roll each half into an 8x12-inch rectangle about ¼-inch thick. Spread half of the meringue on top of each rectangle. Working carefully, roll each rectangle into a long, tight log. Transfer the logs to the baking sheets and chill for 30 minutes.
Heat oven to 350 degrees. Slice the chilled logs into ½-inch slices and place the slices cut side down on the prepared baking sheets, leaving 1-inch between each piece in case of expansion. Bake for 12-15 minutes or until the edges turn a very pale golden brown.