Grease a square 8x8 or 9x9-inch pan with butter/fat, or line with parchment paper, then make sure to also grease the parchment paper so it doesn't stick. Combine the cornstarch and powdered sugar in a bowl or a flour shaker and set aside for later.
Heat sugar, 120 ml of water, and honey/syrup together in a pan to 115 C or 240 F. This will take about 10-15 minutes, so move on to the next steps in the meantime.
Bloom the gelatin powder in 120 ml of cold water. If using gelatin sheets, let them soak in water for 10 mins, then squeeze out excess water and melt in a saucepan with the 120 ml of water until completely liquid.
Transfer the gelatin to the stand mixer and start whisking. Then stream in the sugar syrup, down along the side of the bowl (not directly into the middle of the bowl).
Whisk until really light and fluffy (10-12 minutes), adding the vanilla and cardamom once it’s looking fluffy (about 5 minutes in should be fine). It should come away from the bowl in what looks like ropes.
Pour into the prepared pan, working quickly before it starts to set, and tap it a few times against the counter to make sure it’s even.
Dust the top of the marshmallows with the cornstarch and powdered sugar mixture you prepared earlier.
Let them set overnight at room temperature, uncovered (especially if they’re still a little bit warm).
Tip them out of the pan and dust with more of the cornstarch and powdered sugar. Coat a knife with butter and slice into chunks. Coat them in the powder so they don’t stick to each other. Store in an airtight container for 3-4 weeks at room temperature.