Toast the sesame seeds in a nonstick pan over medium heat until fragrant (about 2 minutes). Transfer the seeds to a small bowl. Cover the seeds with olive oil. Stir once and let them infuse for about 15 minutes. You can prepare this a day ahead; refrigerate the oil until it’s needed.
Meanwhile, in a large Dutch oven, melt the butter over medium-high heat, then add the shallots and stir them constantly until they soften (about 2 minutes).
Add the cauliflower, potatoes, and bay leaves. Add enough of the stock to cover them and bring the mixture to a boil. Cover, reduce heat, and cook on a low boil until the cauliflower is completely tender (about 20 minutes). Heat the remaining stock in a small saucepan until it simmers.
Discard the bay leaves then transfer the vegetables with a slotted spoon to a blender large enough for the soup. Puree the vegetables. Add the stock, a little at a time, until you get a completely smooth consistency. If the soup is too thick, whisk in additional hot vegetable stock, ½ cup at a time, until you've reached the desired consistency. Add the olive oil, a little at a time, to emulsify and make the soup even smoother. Season to taste with salt and pepper. Serve the soup topped with sesame-infused oil.
