In a medium bowl, whisk together the flour, baking powder, baking soda and salt. Set aside.
2 1/2 cups (300g) all-purpose flour, 1 Tbsp baking powder, 1/2 tsp baking soda, 1/4 tsp salt
In a large bowl whisk together the melted butter and sugar until they’re well combined.
1/2 cup (113g) salted butter, 1 1/4 cup (250g) granulated sugar
Add in one egg and whisk until it’s well combined and has lightened in color, about 30 seconds. Repeat with the next two eggs, one at a time.
3 large eggs
Add in the Greek Yogurt and the vanilla extract and whisk until it’s completely smooth.
3/4 cup (170g) Greek Yogurt, 2 tsp pure vanilla extract
Add in the heavy whipping cream and whisk until it’s completely smooth.
3/4 cup (6oz) heavy cream
Add in the dry ingredients and mix just until there are only a few streaks of flour left.
Add in the chopped hazelnuts and mix just until they're evenly distributed and there are no more streaks of flour left.
1/4 cup (40g) finely chopped hazelnuts
Cover with a lid, plastic wrap or a tea towel and set aside to rest at room temperature for 1 hour.
Preheat the oven to 425F (218C) convection (400F/204C conventional) and line 12 muffins tins.
Use a small cookie scooper to fill the muffins tins halfway. Then microwave the Nutella for 15-30 seconds so it's a little more runny. Drizzle a little onto each muffin. Top with another scoop of muffin batter and another drizzle of Nutella. Use a knife to lightly swirl the Nutella into the muffin batter but don't over-swirl it so you don't lose too much definition.
1/2 cup (150g) Nutella spread
Bake the muffins for 5 minutes and then reduce the oven temperature to 375F (190C) and bake for another 13-15 minutes, just until the muffins are golden brown and spring back when you press on the center gently.
Remove from the oven and allow to cool for at least 10-20 minutes before serving.