Line an 8" x 8" baking pan with parchment paper, lightly spray with non-stick oil, or grease with vegan butter. I like to use 2 strips of parchment paper like a "t" to lift the treats out easily.
In a large pot, melt the 3 tablespoon vegan butter over medium-low heat. Add in the 10 oz bag vegan mini marshmallows and stir continuously until they are completely melted together into one large melted marshmallow. It takes about 8 minutes. As soon as they become 1 mass, remove the pot from heat and stir in the 2 teaspoon ube extract.
Add in the 4 cups rice krispie cereal and stir gently until they are completely coated with marshmallow. Transfer to the prepared pan and use your hand and a piece of parchment paper to press down to even them out.
Let the rice krispie treats cool before cutting.
Store in an airtight container for up to 3 days. Keep them at room temperature as they don't do well in the fridge or freezer.
Notes
Ube extract is both a flavoring and a dye. Be careful of stains.
Technically Kellogg's Rice Krispies aren't vegan. They contain vitamin D3 which is made from animals. You can find other brands that are vegan and work well like Nature's Path and store brands.
If you can't find vegan mini-marshmallows, cut up regular sized ones in smaller pieces with scissors.
Make sure you don't brown the butter or the marshmallows! Cook them on low and remove them from heat as soon as they are fully melted.