Whisk the oil, sugar, and eggs until the volume increases slightly.
Mix in the orange juice and zest. If you’re adding orange blossom water to the batter, now is the time.
Mix in the ground pistachios, flour, and instant vanilla pudding powder; stir gently and then add the baking powder. Mix only until combined; do not overmix.
Take out 1¼ cups of the batter and mix it with ¼ cup of pistachio paste in a small bowl.
In a greased pan, add a layer of the cake batter, and in the middle, add a few tablespoons of the green batter.
Repeat the process until all the batter is in the pan.
Using a butter knife, insert it into the batter and run it down the center lengthwise once. Do not overmix the batter, or you will lose the distinct marble effect. *See the recipe notes for instructions if you want to use only one bowl.
Crumble
Mix all the crumble ingredients together with your fingers until combined, and then sprinkle them on top of the cake.
Baking
Bake for 55 minutes to 1 hour, or until a skewer inserted into the center of the cake comes out clean or with a few crumbs.
*Make sure to check after about 30 minutes to see if the cake is already browned. If so, place parchment paper on top to cover it and continue baking.
Notes
*To use just one bowl, after preparing the batter, pour almost all of it into the pan, reserving about 1 ¼ cups to mix with the pistachio paste. Once mixed, add the green pistachio batter to the pan, placing it lengthwise in the center over the rest of the batter. This time, you want to "draw" the shape of an '8' twice with the knife lengthwise.*For the pistachio sauce on top, I mixed heavy cream with a little pistachio paste until it formed a thick sauce.