Prep the pan: Grease a 9-inch (23 cm) square pan with cooking spray and line it with parchment paper, allowing the parchment to extend slightly beyond the edges. This will create handles for lifting the treats out of the pan later.

Cook the crispy rice mixture: In a large shallow saucepan over high heat, melt the butter, salt, and milk together. Once melted, lower the heat to medium and add 5 ¼ cups (320g) of the marshmallows and cook, stirring occasionally, until they fully melt. (You can do this step in the microwave: Just use a medium bowl and blast it for 30 seconds at a time.)

Sift the matcha into the melted marshmallows and mix until combined. Allow the matcha/marshmallow mixture to cool slightly, about 1 minute, before stirring in the rice cereal, pretzels, and vanilla. (This brief cooling period helps prevent the treats from becoming soggy by allowing some steam and moisture to escape.) Stir in the remaining 1 ¾ cups (100g) marshmallows using a stiff spatula, wooden spoon, or, my personal favorite, a fiberglass spatula while the mixture is still warm.

Mold, cut and serve: Transfer the mixture to the prepared pan and press it down flat with an oiled hand or spatula. Let it cool for several hours before cutting. For best results, use an oiled serrated knife and a ruler to cut the treats into even shapes. They're delicious no matter how you slice them. Store airtight at room temperature for up to 3 days.