1. Preheat your oven to 350°F or 180°C and grease a 9x13-inch long pan, which is roughly equivalent to a 23x33-centimeter pan.

2. In a large mixing bowl, whisk together 250g or 2 1/4 cups of flour, 200g or 1 cup of sugar, 1 1/2 teaspoons of baking powder, and 1/2 teaspoon of salt.

3. In another bowl, whisk 3 large eggs and 115g or 1/2 cup of softened butter until smooth. Add 1 teaspoon of vanilla sugar and mix well.

4. Gradually add the dry ingredients to the wet ingredients and mix until a smooth batter forms.

5. Melt 250g or 1 cup of Nutella in a double boiler or in the microwave in 30-second increments, stirring between each interval until smooth. Allow it to cool slightly.

6. Pour the cake batter into the prepared pan and smooth the top.

7. Drizzle the melted Nutella over the cake batter, creating a marbled effect.

8. Bake for 35-40 minutes or until a toothpick inserted in the center comes out clean.

9. Remove from the oven and let cool completely in the pan before slicing and serving.