Preheat oven to 350 degrees Fahrenheit and grease the 8x8 cake pan with canola oil.
Melt your 1/2 cup of butter in the microwave for about 1 minute. While waiting for the butter to melt, crush the graham crackers in a Ziploc bag until it looks like it has the consistency of sand.
In your mixing bowl, combine the crushed graham crackers and melted butter. Use the silicone spatula to mix until the graham crackers absorb the butter. Then press in the buttery graham cracker crust into your baking pan.
Evenly spread the 1/2 can of sweetened condensed milk over the crust and set to the side.
Melt 1/2 bar of white chocolate with 3 tablespoons of butter in a saucepan over low-medium heat. Mix with your spoon. Once it gets to a liquid consistency, add in your 1 cup of mini marshmallows and continue to mix.
Slowly add in a 1/2 cup of the funfetti cake mix into the saucepan and stir.
The cake mix may appear to look fluffy, so once the cake mix is stirred thoroughly, add in your 1/4 cup of milk/non-dairy milk (I used almond milk) and continue to stir.
After the cake batter is melted, pour the mixture over the graham cracker crust.
Pour your 1 cup of semi-sweet chocolate chips over the cake batter filling. Make sure the chocolate chips are spread evenly.
Then carefully drop in the 1/2 cup of white chocolate chips over the semi-sweet chocolate chips.
Once your chocolate chips are all in, place the pan in the oven for 20-25 minutes at 350 degrees Fahrenheit.
When your timer goes off, remove the pan from the oven and let it cool for a few minutes.
Using your decently-sharp knife, cut your marshmallows in half and place on top of your bars. I ended up using about 8.5 marshmallows total!
This part is VERY important. Place the pan into the oven and set to Broil. I had my broiler set at 545 degrees Fahrenheit. Watch the marshmallows like a hawk! Do not look away for more than 2 minutes, unless you want burnt marshmallows; But that's not the look we're going for! We are trying to achieve golden perfection with these marshmallows!
Once the marshmallow topping is all toasted and golden and beautiful, remove from the oven and let it cool. I would recommend that you refrigerate them for 1 hour, but if you want them to still have that ooey, gooey s'mores texture, you can let them cool for 10-15 minutes and serve warm! Just make sure they aren't too hot before consuming these golden beauties!
You can always substitute the funfetti cake mix for normal yellow cake mix!
Alternatively, if you're not feeling the cake batter flavor, you can always exclude that step and just add in more chocolate chips for that classic s'mores taste!