In a large saucepan, bring the stock, corn cobs and 2 cups water to a simmer; season with salt and pepper. In a blender, puree the scallion greens, cilantro, jalapeno, lime juice and 5 tbsp. oil. Transfer the pesto to a small bowl; season.
In a large pot, heat 1 tbsp. oil over high. Add the corn kernels and cook, stirring often, until golden, 3 to 5 minutes; season. Transfer half the kernels to another small bowl. Rinse the blender. Puree the remaining kernels and 3 tbsp. corn stock; season.
Wipe the pot clean and heat the remaining 1 tbsp. oil over medium. Add the rice and scallion whites and cook, stirring often, until coated, about 2 minutes. Add the corn stock, 1 cup at a time, stirring frequently and allowing the liquid to absorb between each addition, until the rice is tender but still firm and the risotto is creamy, about 20 minutes. Stir in the corn puree, lime zest and butter; season. Top with the reserved corn kernels; drizzle with the pesto.