To make the cake: Heat oven to 200 degrees F. Line two baking pans with parchment paper and set aside. Draw two 9-inch circles in the center of the paper and set aside.
Beat egg whites and cream of tartar until foamy. Stream in 1 1/4 cups sugar and continue to beat on high speed. Just as the whites reach their peaks, sprinkle in sifted cocoa and cayenne pepper and continue beating to stiff peaks.
Spread the meringue within the circles. Bake for 3 hours, turn off oven, and let sit for 1 hour.
Cool on the pan on a wire rack. Store in a very dry, cool place.
Fill the layers with whipped cream and serve within 2 hours.
Fill two mugs with hot water and let sit 2 minutes. Pour out water and add 1 tsp light brown sugar to each mug. Pour over hot coffee and stir to dissolve sugar, then pour in Jameson.
In a separate bowl with an electric mixer or by hand, whisk cream until soft peaks form. Cream should be thick but still pourable. Top coffee with cream by gently pouring over the back of a warm spoon to form a thick layer on top of coffee. Garnish with chocolate shavings.