1tbspinstant coffee powderdissolved in ½ tbsp hot water ( let cool)
¾cupflour
½tbspbaking powder
4egg whites
¼tspcream of tartar
¼cupsugar
For the Mocha Swiss Meringue Buttercream
2egg whites
½cupsugar
1stick softened butterunsalted
1tbspinstant coffee powderdissolved in ½ tbsp hot water ( let cool)
Instructions
Make the Cake
Line a 15 x 10 jelly roll pan with parchment paper. Preheat oven to 370 F.Sift together flour and baking powder in a bowl and set aside. Whisk the egg whites using high speed of stand mixer. Once frothy, add in the cream of tartar. Once soft peaks form, gradually add the ¼ cup of sugar until the egg whites form shiny, stiff peaks. A good indicator: turn the bowl upside down and the egg whites should not spill or flow.
Meanwhile, beat together the egg yolks and sugar using a wire whisk. Once it is thick and light colored, add in the milk, vegetable oil and the coffee . Add in the flour mixture and whisk together until the mixture is smooth.
Gently fold the egg whites into the yolk batter in gradual amounts using a rubber spatula. Pour batter into the jelly roll pan and spread evenly to all sides using a spatula.Bake at 370 F for 8-10 minutes or until the cake springs back to the touch. A toothpick inserted unto the center of cake should come out clean. Turn the pan down into a kitchen towel dusted with powdered sugar. The top of the cake should now be facing down.Carefully peel of parchment paper from the cake.
Lift the short side of the cake and start rolling it inwards together with the towel. Place seams down and let cool. Gently unroll the towel so that the cake is flat on the surface.Spread filling on top. Using the towel, lift the short side of cake again and roll inwards, let go of the towel as you roll. Trim the edges to make them straight.
For the Mocha Swiss Meringue Buttercream
Fit a heat proof glass bowl on top of a pot with simmering water. Add the egg whites and sugar and start whisking continuously using a wire whisk. Once the mixture turns frothy and white, and reaches a temperature of 160 F, turn off heat. That should take about ten minutes or so.
Pour the egg whites mixture into the bowl of a stand mixer fitted with the whisk, and start whisking on high speed until the mixture forms shiny and firm peaks. The bowl should be cold to the touch. Add in the softened butter to the mixer and continue whisking. The buttercream might turn lumpy or might curdle for a moment. Keep whisking until a thick, fluffy icing takes form. Add in the coffee mixture and continue whisking until combined. Store in a tightly closed container or use icing right away.