Preheat the oven to 350°F / 175°C. Butter and line an 8×4" loaf pan with parchment paper.
Brew the espresso. In a medium sized mixing bowl, whisk together the cocoa and hot espresso.
Add in the oil, sugar, eggs, and sour cream. Whisk vigorously until well combined.
Add in the flour, baking powder, and salt. Whisk together just until no flour streaks remain.
Pour the chocolate cake into the prepared loaf pan. Bake for 50-60 minutes, or until a toothpick inserted in the center of the cake comes out clean or with only a few moist crumbs.
Filling
Allow the cake to fully come to room temperature before assembling it. Once the cake is at room temperature, take out the ice cream to sit at room temperature for ~20 minutes to soften.
Slice the cake horizontally twice to get 3 pieces. I recommend making the top layer of the cake a little thicker to allow more room for ice cream in the pan.
*important!* Line the loaf pan you took the cake out of completely with plastic wrap to form a hammock for the cake. This will make it way easier to pull the cake out of the loaf once it's completely frozen later on!
Put the bottom layer of the cake into the loaf pan. Spread half of the chocolate ice cream carefully over the cake.
Add half the nutella and half of the hazelnuts on top of the ice cream and swirl them around into the ice cream.
Gently top it with the middle cake layer, and carefully spread the rest of the ice cream over it. Swirl in the remaining nutella and hazelnuts.
Top it off with the top layer of the cake. Bring the plastic wrap up from the sides to cover the top of the cake, and top with additional plastic wrap as needed just to fully cover the loaf.
Place the cake in the freezer for at least 6 hours, ideally overnight before slicing.
Nutella Whipped Cream
Once you're ready to serve the ice cream cake, remove the cake from the freezer to soften for ~20 minutes, and make the nutella whipped cream.
Add the heavy whipping cream, salt, sugar and nutella to a medium sized mixing bowl. Beat the mixture with a hand or stand mixer fitted with the whisk attachment until soft peaks form.
Top the ice cream cake with the whipped cream and additional hazelnuts and chocolate chips, if desired. Slice, serve, and enjoy!