Line an eight-inch square baking pan with aluminum foil, gently pressing the foil into the corners and letting the extra foil hang over the sides. Soak a paper towel with the vegetable oil and use it to rub over the foil. Set aside 1/2 cup of the mini marshmallows and 1/2 cup of the nuts.

Put the chocolate chips and condensed milk in a medium microwave-safe bowl. Microwave the mixture on high for one minute. Stir with a rubber spatula. If the chips aren’t melted, return to the microwave one or two times for 30 seconds each, stirring after each time, just until the chocolate is melted. Don’t let the chocolate get too hot!

Using a rubber spatula, gently stir the vanilla, the remaining three cups of mini marshmallows, and the remaining one and a half cups of the nuts into the chocolate mixture. Using the spatula, scrape the chocolate mixture into the prepared baking pan. Spread it evenly and smooth the top.

Sprinkle the fudge with the reserved marshmallows and nuts and gently press them into the surface. Cover the pan with plastic wrap and refrigerate until firm, at least 30 minutes.

Holding the ends of the foil, lift the fudge out of the pan and set it on a cutting board. Peel away the foil and cut the fudge into 16 squares.