½tspcoarse Kosher salt(if using table salt, use half the amount)
1 ½tspbaking powder
3ozwhite chocolate (1/3 cup white chocolate chips),melted and cooled slightly
1 ½cupsunsalted butter,softened to room temperature
⅛tspcoarse Kosher salt(if using table salt, use half the amount)
¾tspfresh orange zest
1-2tbspmilk or cream,only if needed for consistency
Toast the Hazelnuts
Preheat the oven to 325 F, and spread the hazelnuts into a single layer on a baking sheet.
Toast the hazelnuts in the oven for about 10-12 minutes, until hot and fragrant.
Wrap the hot hazelnuts in a clean dish towel, and let them steam in the towel for 1 minute, then rub the towel over the hazelnuts to loosen as much of the papery skin as you can. If any skin is still clinging to the nuts, that's fine.
Let cool, then use a small food processor to finely chop the nuts.
Brown the Butter
In a saucepan, melt the 1/2 cup butter over medium heat. Continue to cook the butter, swirling the pan occasionally, until fragrant and nutty brown solids form at the bottom of the pan. A layer of foam might form on top of the butter, and this is usually when the butter is ready.
Use a spatula to scrape all the golden brown solids off the bottom of the pan, then pour the browned butter into a large mixing bowl.
Make the Cake
Preheat the oven to 350 F, and position a rack in the center of the oven. Grease three 6-inch cake pans with non-stick baking spray.
In a large mixing bowl, whisk together the hot browned butter with the oil, granulated sugar and brown sugar until smooth. Whisk in the eggs, milk, sour cream, vanilla extract and hazelnut extract.
In a separate bowl, whisk together the flour, salt, baking powder and chopped hazelnuts.
Add the dry ingredients to the liquid ingredients and whisk just until smooth.
Divide the cake batter between the greased pans and bake for about 19-20 minutes, until a cake tester or toothpick inserted in the center comes out clean.
Set the pans on a cooling rack, covered loosely with a clean kitchen towel, and cool completely before frosting.
Make the Buttercream
In a double boiler or in the microwave at 50% power, melt the white chocolate until smooth. Set aside to cool for 10-15 minutes until cooled to room temperature, but still smooth and drizzly.
In the bowl of your stand mixer fitted with the whisk attachment, beat the butter for 1 minute until smooth.
With the mixer on low, add the powdered sugar by spoonfuls, the meringue powder, salt and extracts.
Increase speed to medium and whip for 4-5 minutes, scraping the bowl several times, until very light and fluffy. Only add the milk if the buttercream seems too stiff and dry.
Add the orange zest and the melted/cooled white chocolate and whip for 1 minute, then turn the speed down to low and mix for 2 minutes to get rid of air bubbles.
Assemble and Frost the Cake
Remove the cooled cakes from the pans. Place one of the cooled cakes on a cake board or cake pedestal.
Fill, stack and crumb coat the cake, then chill for 30 minutes to set the crumb coat, keeping the bowl of buttercream covered in the meantime. Frost the cake all over with a final coat of buttercream.
If you like, fill a piping bag with any extra buttercream and pipe swirls on top of the cake.