Preheat the oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or grease the cups.
In a large bowl, whisk together the all-purpose flour, granulated sugar, baking powder, baking soda, and salt until well combined.
In a separate medium bowl, whisk together the plain Greek yogurt, vegetable oil, eggs, and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients and gently fold together with a spatula until just combined. Do not overmix; the batter should be slightly lumpy.
Spoon half of the muffin batter evenly into the prepared muffin cups, filling each about one-third full.
Add about one teaspoon of Nutella on top of the batter in each muffin cup.
Spoon the remaining batter over the Nutella layer, filling the muffin cups about two-thirds full.
Using a toothpick or skewer, gently swirl the batter and Nutella together to create a marbled effect.
Bake in the preheated oven for 18 to 20 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Remove the muffins from the oven and let them cool in the tin for 5 minutes, then transfer to a wire rack to cool completely before serving.