Prepare an 8-inch square baking pan by lining it with parchment paper. Using a sieve, generously dust with powdered sugar.
In a large metal/ceramic mixing bowl (no plastic! we will be pouring hot liquid into it later) or bowl of stand mixer, pour the ½ c cold water, then sprinkle on two envelopes of gelatin on top. Set aside to let the gelatin soak up the liquid.
In saucepan, add ½ c water and all of the sugar. Turn heat to medium and let it cook for a few minutes until the sugar fully dissolves.
When the mixture starts boiling, check temperature using candy thermometer. Once it reaches about 240°F/115°C, remove sugar water mixture from heat.
Turn on hand/stand mixer to medium speed and whip the thickened gelatin while slowly adding all of the hot sugar water mixture.
Once all of the sugar water is added, add the ube extract and keep mixing on medium speed. If you want a deeper purple color, add your purple food coloring now.
Whip for about 10-15 minutes, or until the mixture is thick, glossy, doubled in size, and reaches soft peaks. If you overwhip, the mixture will be dull, come apart easily, and hard to spread.
Add all of the marshmallow mixture into your prepared square pan. Level and smooth the top. Dust with more powdered sugar and leave uncovered overnight (or ~8 hr) to allow the marshmallow block to fully set.
Remove marshmallow block from pan and cut into sizes. Dip cut sides into powdered sugar to prevent them from sticking.