Adjust oven rack to upper-middle position and heat oven to 400 degrees F. Spray 18 by 12-inch rimmed baking sheet with PAM baking spray, cover pan bottom with parchment paper and spray parchment with another layer of Pam baking spray.
Combine chocolate and butter in a small bowl. Melt the chocolate in the microwave by heating the chocolate at 50% power for 30 seconds. Stir and repeat until butter and chocolate are melted. Stir until the chocolate is smooth and glossy. While chocolate is melting, sift cocoa, flour, and salt together into a small bowl and set aside.
In bowl of a standing mixer, beat egg yolks at medium-high speed until just combined, about 15 seconds. With mixer running, add half the sugar. Continue to beat, scraping down sides of bowl until yolks are pale yellow and thickened, about 8 minutes. Add vanilla and beat to combine. Place mixture into medium bowl; wash and dry mixer bowl and beaters.
In clean bowl, beat egg whites and cream of tartar at medium speed until foamy, about 30 seconds. With mixer running, add about 1 teaspoon of the remaining sugar; continue beating until soft peaks form, about 1 minute. Gradually add remaining sugar and beat until whites are glossy and supple and hold stiff peaks when whisk is lifted, about 1-2 minutes longer. DO NOT over beat (if whites look dry and granular, they are over beaten). While whites are beating, stir chocolate mixture into yolks. With rubber spatula, stir a quarter of the whites into chocolate mixture. Fold in remaining whites until almost no streaks remain. Sprinkle dry ingredients over egg and chocolate mixture and fold in gently.
Pour batter into prepared pan; using offset icing spatula and working quickly, smooth surface and spread batter into pan corners. Bake until center of cake springs back when touched with finger, 8 to 10 minutes, rotating pan halfway through baking time. Cool cake in pan on wire rack for 5 minutes.
While cake is cooling, lay a clean kitchen towel over work surface and sift 1 tablespoon cocoa powder over towel. With your hands, rub cocoa into towel. Run knife around the edges of baking sheet to loosen cake. Invert cake onto towel and peel off parchment.
Starting at a long side, roll cake and towel together like a jelly roll. Cool for 15 minutes. While the cake cools, make the peppermint buttercream by adding the butter, cream, and confectioner's sugar together in a medium bowl. Using a handheld mixer, beat until smooth. Add the peppermint baking chips and mix until just combined.
At the end of the 15 minutes unroll the cake and towel. Using an offset spatula, immediately spread the peppermint buttercream evenly over the cake, almost to edges. Re-roll cake gently but snugly around filling. Set large sheet of parchment paper on overturned rimmed baking sheet and set cake seam-side down on top. Trim both ends of cake.
Make the ganache by microwaving the bittersweet chocolate and cream in a small bowl at 50% power for 30 seconds. Stir and repeat until butter and chocolate are melted. Add confectioner's sugar and mix to combine. Add water 1 tablespoon at a time, mixing after each addition until pouring consistency is reached. Set aside and let sauce cool to warm.
Spread ganache over roulade with small icing spatula. Sprinkle peppermint baking chips over the top and refrigerate cake, uncovered, to slightly set icing, about 20 minutes. Serve.