1/4 cup milk
2 tbsp butter
3/4 cup sifted cake-and-pastry flour
1 tsp baking powder
1/4 tsp salt
3/4 cup granulated sugar
1 tbsp icing sugar
1 cup butter, softened
1/3 cup whipping cream
2 tsp vanilla
2 cups icing sugar
4 oz unsweetened chocolate, melted and cooled
Line 16x10-inch (40x25-cm) rimmed baking sheet with parchment paper.
In bowl over saucepan of simmering water, heat milk with butter until melted. Remove pan from heat, leaving bowl on top to keep warm enough that finger can remain in bowl for no longer than 10 seconds.
In separate bowl, whisk together flour, baking powder and salt; set aside.
Separate yolks and whites of three of the eggs into separate bowls. Beat egg whites until foamy; beat in 1/4 cup (60 ml) of the granulated sugar, about 1 tbsp (15 ml) at a time, until soft peaks form.
Beat egg yolks, remaining eggs and granulated sugar until thick enough that batter leaves ribbons on surface when beaters are lifted, about five minutes. Fold in one-third of the whites; fold in remaining whites. Sift flour mixture over top; fold in. Pour in milk mixture; fold in just until blended. Spread in prepared pan.
Bake in centre of 350 F (180 C) oven until golden and cake springs back when lightly touched, about 12 minutes. Loosen edges with knife; place flour-dusted tea towel over cake. Top with tray larger than pan. Invert and lift off pan. Starting at corner, peel off paper. Starting at short side, roll up cake in towel; let cool on rack. (If making ahead, store in airtight container for up to 24 hours.)
Unroll cake; spread with 1 1/2 cups (375 ml) of the icing. Re roll without towel; place, seam side down, on flat serving platter. Spread remaining icing all over log. Arrange Chocolate Bark over log.
Refrigerate until cold. Dust bark with icing sugar.