Step 1Put the potatoes in a large saucepot and add enough cold water to cover by 2 inches. Add 3 tablespoons salt. Cover and bring to a boil over high heat. Reduce the heat to keep the potatoes boiling without causing the water to spill out. Continue cooking until the potatoes are very tender, about 15 minutes. A knife should slide through very easily and cause the potato to split.
Step 2While the potatoes boil, combine both oils, the leeks and a generous pinch of salt in a large saucepan. Set over medium heat and cook, stirring occasionally, until the leeks are very tender, about 15 minutes. Add the nutritional yeast and cook, stirring for 1 minute, then add the broth. Bring to a boil, then simmer for 10 minutes.
Step 3Drain the potatoes and let stand in the colander until very dry, about 5 minutes. Pass the potatoes through a food mill or ricer back into the pot in which they cooked.
Step 4Transfer the leek mixture to a blender and puree on high speed until very smooth. Add 2 cups of the leek mixture to the potatoes and fold gently until evenly distributed. Season to taste with salt and pepper. For creamier potatoes, fold in more of the leek mixture. Transfer to a serving dish, drizzle with more olive oil and garnish with chives.