Preheat the oven to 350F/177C convection (325F/163C conventional) and line 13 muffin tins with cupcake liners (preferably a light colored pan).

In a medium bowl, or the bowl of a stand mixer, whisk together the cake flour, baking powder, salt and sugar.

Add the oil and butter to the dry ingredients and use an electric mixer to beat them together until they're combined.

Add the egg and vanilla extract and mix until they're fully combined.

Then add the sour cream and milk, mixing just until the batter is smooth. 

Fill each cupcake liner about 2/3-3/4 full (about 50g). Bake for 16-18 minutes.

The cupcake should no longer look wet and when you press on the center of the cupcake it should spring back, or once a toothpick inserted comes out clean.

Remove them from the oven and let them cool in the cupcake pan for 10 minutes, then move them to a cooling rack.

Allow the cupcakes to cool completely and then use a cupcake corer or a knife to cut a whole out of the center. Set aside.

While the cupcakes are baking, ground your hazelnuts and place them in a short, wide bowl or on a baking sheet. Set aside.

Place the sugar and water in a sauce pan over medium heat. Stir intermittently until it is fully dissolved and the color is like honey.

Pour the sugar evenly over the hazelnuts and stir quickly before it cools and hardens.

Return the nuts with the sugar to the food processor and pulse until there are no more large chunks. 

Place the sugar and water in a medium saucepan and swirl the pan until the sugar is fully immersed in water.

Place the saucepan over medium heat until the sugar dissolves and comes to a gentle simmer. Lower the heat to low and place the lid on the pan and allow the mixture to simmer for a couple minutes. 

Meanwhile, place the eggs in the bowl of a stand mixer, fitted with the whisk attachment, and turn the mixer up to high speed. Let it whip while the sugar is cooking.

Use a candy thermometer to see when the sugar reaches 240F (116C), then lower the mixer to medium speed and slowly drizzle the hot sugar syrup into the thick and foamy eggs, while they are whipping.

Turn the mixer back up to high speed and let it whip for 5-10 minutes, until the egg mixture is very pale and fluffy and is cool to the touch or no longer warm.

Turn the mixer back down to medium speed and add in the butter, 2 Tbsp of butter at a time, mixing until each chunk is fully combined.

Turn the mixer up to medium-high speed until the buttercream has fully come together.

Scrape the sides and bottom of the bowl, add in the Nutella, vanilla and salt and turn the mixer up to high speed for 1-2 minutes.

If you want to remove the air and make it smooth and creamy instead of fluffy, switch to a paddle attachment and turn the mixer to low speed for 5 minutes.

If you haven't already, use a knife or a cupcake corer to remove the center of each cooled cupcake. Fill the center with Nutella spread.

Use a piping bag fit with a large round piping tip to frost the cupcakes.

Use a piping bag with a tiny cut or a small round piping tip to drizzle the Nutella over the buttercream.

Sprinkle some hazelnut praline on top of the cupcakes.