Mash avocados with cilantro, shallots, lime juice (start on the low end of the range), and salt. Add more lime juice and / or salt to taste. If you’d like it spicy, stir in some hot sauce. Set aside.
Heat butter in large skillet over medium-high heat until melted. Add mushrooms and saute until very tender, 5 to 7 minutes. Season with some salt. Transfer cooked mushrooms to a bowl and return skillet to heat.
To heated skillet, add oil and then corn. Cook corn, without stirring, until it begins to pop and char on the bottom, 4 to 5 minutes. Stir and continue to cook until corn is very tender, about 2 minutes more. Season with some salt. Transfer to bowl with the corn.
Heat tortillas according to package directions (If you have a gas stove, you can lightly toast them over the gas or just heat them all in the microwave).
Spread tortillas with guacamole. Fill with mushroom-corn mixture and cotija. Garnish with cilantro leaves.