My daughter discovered these on spring break and we have made them every weekend since.

Line your slow cooker with a large piece of parchment paper, letting it come up the sides so you can lift the blondies out later. Lightly mist the parchment with nonstick spray if you have it; this helps the edges release cleanly.
In a large mixing bowl, whisk the dry cake mix and matcha powder together until the color is evenly bright green with no dark streaks. Take a minute to really blend it well so the final blondies bake up an even, vibrant color.

Pour in the sweetened condensed milk and melted butter. Stir with a sturdy spatula or wooden spoon until a thick, sticky batter forms. It will be quite dense—more like cookie dough than cake batter, and that’s what gives you the chewy, blondie texture.
Fold in about 3/4 cup of the white chocolate chips or chunks, reserving the rest for the top. Mix just until the pieces are evenly distributed throughout the green batter.
Scrape the batter into the parchment-lined slow cooker. Use the spatula or lightly damp fingers to press it into an even layer, reaching all the way to the edges. Sprinkle the remaining white chocolate chips or chunks over the surface and gently press them in so they stick but still show on top.

Cover the slow cooker with its lid. If your lid tends to drip condensation, you can lay a clean kitchen towel or a couple of paper towels over the top of the crock (not touching the batter), then place the lid on to catch excess moisture. This helps keep the top of the blondies lightly crackled instead of wet.
Cook on LOW for 2 to 2 1/2 hours, or until the edges are set and slightly pulling away from the sides, the top looks dry with tiny cracks, and the center is just barely set but not jiggly. Every slow cooker runs differently, so start checking around the 2-hour mark. Avoid lifting the lid too often, as that releases heat and can lengthen the cook time.
Once done, turn off the slow cooker and remove the lid. Let the blondies cool in the slow cooker for about 20–30 minutes to firm up slightly. Then use the parchment paper as handles to carefully lift the entire slab out onto a cooling rack or baking sheet.

Cool completely at room temperature, at least 1–2 hours, before cutting. This cooling time is what gives you those dense, fudgy, clean-cut squares with a chewy bite and a slightly crackled top. Use a sharp knife to cut into squares, wiping the blade between cuts for neat edges.
Store leftover blondies in an airtight container at room temperature for up to 3 days, or refrigerate for a firmer texture. Bring chilled blondies back toward room temperature before serving, or warm them for a few seconds in the microwave if you like them extra gooey.
Variations & Tips