Melt 1 tablespoon butter in a wide sauté pan over medium-low heat. Add 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 rice has absorbed all of the stock. Continue cooking, adding cups of stock whenever 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, corn is tender and rice is creamy and tender, about 30 to 40 minutes total.
Remove risotto from heat and stir in Parmesan and remaining tablespoon of butter. Cover and let stand for 5 minutes. Uncover risotto, stir and season to taste with salt and pepper. Immediately before serving, stir in the mascarpone cheese. Top with grilled shrimp and a spoonful of salsa verde.