Dessert: Bûche de Noël

Recipe by /contributors/claire-saffitz Of all the quirky, inexplicable, reindeer-embellished holiday traditions out there, making your own Yule log might take the cake. Why would anyone in their right mind create a dessert that is supposed to look like s

Ingredients

6 large egg whites
1 1/4 cups sugar
1/4 teaspoon cream of tartar
1/4 teaspoon kosher salt
Unsweetened cocoa powder (for dusting)
1 cup (2 sticks) unsalted butter, cut into 1" pieces, room temperature
1 teaspoon vanilla extract
1 vanilla bean, split lengthwise

Instructions

Preheat oven to 225 °F. Whisk egg whites, sugar, cream of tartar, and salt in a large heatproof bowl set over a medium saucepan of simmering water (bowl should not touch water) until egg whites are very liquid (mixture will be warm to the touch) and sugar is dissolved (rub between your fingers to check), about 5 minutes. Remove bowl from heat and beat with an electric mixer until stiff, glossy peaks form (bowl will feel cool), about 5 minutes.
Line a rimmed baking sheet with parchment paper and dust very lightly with cocoa powder. Scoop out 1 cup meringue. Dollop heaping teaspoonfuls of meringue onto prepared baking sheet, spacing at least 1 1/2" apart.
Lightly dust meringue with more cocoa powder and place a sheet of parchment over top. Use an offset spatula or your hands to flatten meringue into irregularly shaped disks about 1/8" thick. Bake until lightly browned and parchment peels away easily, 75 –90 minutes (meringue will be soft when it first comes out of the oven but will dry and crisp as it cools).
Meanwhile, with mixer on medium speed, beat butter into remaining meringue, adding a piece at a time and waiting until incorporated before adding more. Beat until mixture is very smooth. (If it looks very loose or curdled, just keep beating; buttercream will come back together.) Add vanilla extract and scrape in vanilla seeds; beat to combine.