Melt 1 tablespoon butter in a wide sauté pan over medium-low heat. Add the leek and shallot and cook, stirring occasionally, until softened but not browned, about 6 minutes. Season with salt and pepper. Add rice and cook, stirring, until grains look slightly translucent.
Pour in wine and cook, stirring, until it has all been absorbed, about 2 minutes.
Add a ½ cup of stock to the rice mixture and cook, stirring constantly, until the rice has absorbed all of the stock. Continue cooking, adding ½ cups of stock whenever the rice mixture looks dry and stirring continuously. When half the stock has been added, stir in corn. Continue cooking until all of the stock is incorporated, the corn is tender, and the rice is creamy and tender, about 30 to 40 minutes total.
Remove risotto from heat and stir in Parmesan and the remaining tablespoon of butter. Cover and let stand for 5 minutes. Uncover the risotto, stir, and season to taste with salt and pepper. Immediately before serving, stir in the mascarpone cheese. Top with shrimp and a spoonful of salsa verde.