Preheat oven to 325°F and set aside a greased 8-inch square baking pan or 9-inch round baking pan.

In a large bowl, whisk together all-purpose flour, sugar, cocoa powder, baking soda, baking powder, and salt. Set aside.

In a separate bowl, whisk together coffee, oil, milk, vanilla, and egg.

Add liquid ingredients to flour mixture, and whisk until thoroughly combined. Transfer batter to prepared baking pan and bake until an inserted toothpick near the center comes out clean, about 30 to 40 minutes. Cool in the pan for 15 minutes, then transfer to a wire rack to cool completely.

In a large bowl, whisk whipping cream on medium high speed until stiff peaks form. Add Nutella and gently fold until thoroughly combined and no streaks remain.

If desired, transfer mousse to a piping bag.

Cut cooled cake into bite size pieces.

Divide half of the cake pieces amongst the serving dishes (or a single dish). Pipe or spoon half of the mousse over the cake. Top with remaining cake and mousse. Garnish as desired.

Trifles must be kept refrigerated until ready to serve.