Line your square pan with parchment paper to make removal easy. Gather your saucepan, spatula, and other tools before starting.
In the saucepan, melt the butter over low heat until it shimmers softly, filling your kitchen with a rich, buttery aroma. Stir occasionally to encourage even melting, about 2 minutes.
Add the dark chocolate chips to the melted butter. Stir gently with a spatula until the mixture is smooth, glossy, and fully melted, about 3 minutes. You should hear a gentle bubbling and see a shiny, uniform chocolate sauce.
Stir in the mini marshmallows and vanilla extract. Continue stirring until the marshmallows are just starting to melt and the mixture is thickened and gooey, about 2 minutes. The marshmallows will create a stretchy, shiny texture that smells sweet and chocolatey.
Remove the saucepan from heat and quickly fold in the chopped nuts. Mix until evenly distributed, ensuring nuts are coated in the glossy chocolate mixture.
Pour the thick, fudgy mixture into your prepared pan. Use the spatula to press it evenly into the corners and smooth the top with gentle pressure, creating a rustic, slightly textured surface. This step helps compact everything for better slices.
Let the mixture cool at room temperature for about 15 minutes, then transfer the pan to the fridge. Chill for at least 2 hours until the mixture is firm, crackly at the edges, and holds a clean cut when pressed.
Once chilled and set, lift the block out of the pan using the parchment paper. Place on a cutting board and slice into 1-inch squares with a sharp knife, wiping the blade between cuts for clean slices.
Arrange the chews on a serving plate. These sticky, chewy treats are best enjoyed slightly chilled, paired with a glass of milk or your favorite coffee for a nostalgic snack.
