Place whole oranges in a pot and cover with water.
Simmer for 45 to 60 minutes until soft.
Let cool, then remove seeds if needed.
Blend oranges into a smooth puree.
Preheat the oven to 350°F and grease a cake pan.
In a bowl, whisk eggs and sweetener until combined.
Add orange puree and vanilla extract, then mix well.
Stir in ground pistachios, baking powder, and salt.
Mix until smooth.
Pour batter into the prepared pan.
Bake for 35 to 40 minutes until the center is set.
Let cool completely before slicing and serving.
