Tiramisu is a popular dessert choice around the holidays – the combination of cream and cake with subtle hints of coffee and cocoa make for a winning combination and shaped into classic Yule log, this will be a double hit.
Preheat the oven to 350ºF (180ºC) and line the bottom of a 10-x-15-inch jelly roll pan (a baking tray with a lip) with parchment paper.
Whisk the egg yolks with the icing sugar, vanilla and coffee powder by hand in a large mixing bowl until the sugar and coffee powder has dissolved. In a separate bowl and using electric beaters or in the bowl of a stand mixer fitted with the whip attachment, whip the egg whites and cream of tartar on high speed until foamy and then slowly pour in the granulated sugar and continue to whip until the whites hold a medium peak when the beaters are lifted.
Sift 1/4 cup (35 g) of the flour into the yolk mixture and whisk in. Fold the whipped whites into the yolk mixture in two additions. Sift the remaining 1/2 cup (65 g) of the flour over the batter and quickly but gently fold in. Spread the batter evenly in the prepared pan and bake for 12 to 14 minutes, until the cake springs back when gently pressed with your fingers.
Let the cake cool for no more than 5 minutes on a rack, then use a palette knife to loosen the cake from the edges of the pan. Lightly dust the cake with icing sugar and flip the cake out onto a clean tea towel and peel away the parchment paper. Dust this side with icing sugar and then use the tea towel to lift and roll up the warm cake starting from a short side (the towel should be rolled with the cake). Let the cake cool completely to room temperature.
Place the chopped chocolate in a bowl and pour the hot coffee overtop. Let this sit one minute and then whisk until the chocolate has melted. Whip the cream until it holds a soft peak when the beaters are lifted and chill until ready to use. Cream the mascarpone cheese and icing sugar to soften it a little and stir in the vanilla. Stir in the melted chocolate and then fold in the whipped cream.
Unroll the cooled sponge cake. Spread the mascarpone filling over the cake and carefully re-roll the cake. Transfer this to a serving platter, or wrap in plastic, and chill the cake for at least 2 hours (but it can be assembled a day ahead of serving).
Whip the cream on high speed with electric beaters or in a stand mixer fitted with the whip attachment until it holds a peak when the beaters are lifted. Stir in the sugar, skim milk powder and coffee powder, and let it sit a moment, whisking again to make sure the coffee powder has dissolved into the cream. Spread this over the cake to cover it completely, including the ends. Dust the top of the cake with cocoa powder and chill until ready to serve.
The cake will keep, refrigerated, for up to 3 days.