For cherry filling, stir together the 1/4 cup sugar and the cornstarch in a small saucepan. Stir in the 2 tablespoons water. Add cherries. Cook and stir over medium heat until thickened and bubbly. Cook and stir for 2 minutes more. Remove from heat. Stir in 1-1/2 teaspoons cherry liqueur or brandy. Cover surface with plastic wrap; cool.
For cake, grease and lightly flour a 15x10x1-inch baking pan; set aside. Combine flour, cocoa powder, and baking soda in a small mixing bowl; set aside. Beat egg yolks and 1/2 teaspoon vanilla in a medium bowl with an electric mixer on high speed for 5 minutes or until thick and lemon colored. Gradually add the 1/3 cup granulated sugar, beating on high speed until sugar is almost dissolved.
Wash beaters thoroughly. Beat egg whites in a large mixing bowl on medium speed until soft peaks form (tips curl). Gradually add the 1/2 cup granulated sugar, beating until stiff peaks form (tips stand straight). Fold egg yolk mixture into egg white mixture. Sprinkle flour mixture over egg mixture; fold in gently just until combined. Spread batter evenly in the prepared pan.
Bake in a 375°F oven for 12 to 15 minutes or until top springs back when lightly touched. Immediately loosen edges of cake from pan; turn cake out onto a towel sprinkled with powdered sugar. Roll towel and cake into a spiral, starting from a long edge. Cool on a wire rack.
For chocolate buttercream: Stir together 1/3 cup sugar and 2 tablespoons water in a heavy small saucepan. Bring to boiling. Remove from heat. Gradually stir about half of the sugar mixture into 2 slightly beaten egg yolks. Return all of the egg yolk mixture to the saucepan. Bring to a gentle boil; reduce heat (mixture may look curdled but will become smooth). Cook and stir for 2 minutes more. Remove from heat.
Add 1-1/2 teaspoons cherry liqueur or cherry brandy, if desired. Stir in 1/2 teaspoon vanilla. Cool to room temperature. Beat unsalted butter in a large mixing bowl with an electric mixer on high speed until fluffy. Add cooled egg yolk mixture and melted chocolate, beating until combined. If necessary, chill until mixture is of spreading consistency.
To assemble, unroll cake. Spread cake with chocolate buttercream to within 1/2 inch of edges. Spread cherry filling lengthwise over buttercream layer in a 1-1/2-inch-wide band, starting 1/2 inch from a long edge. Roll up cake without the towel, starting with long edge closest to filling. Wrap in plastic wrap; refrigerate for at least 1 hour or up to 2 hours.
To serve, cut cake roll into 1-inch-thick slices. Garnish slices with chocolate curls, if desired. Makes 15 servings.
Make-ahead tip: Bake, roll, and cool cake as directed. Place cake, with towel, in large airtight container or wrap well with foil. Chill up to 24 hours. Let stand about 30 minutes at room temperature before unrolling and filling.