Lightly dust a non-metal baking dish with sugar free powdered sweetener, coating the bottom and corners, then set aside.

In the bowl of a stand mixer, combine the agar-agar powder and 1/2 cup cold water, stirring until fully moistened, and let it sit to bloom while you prepare the syrup.

In a medium saucepan over medium heat, combine the Splenda, 1/2 cup water, and optional pinch of salt, stirring until the mixture turns clear and begins to gently simmer around the edges, about 8 to 10 minutes.

Remove the saucepan from the heat and immediately stir in the vanilla bean paste until evenly combined and fragrant.

With the mixer on low speed, carefully pour the hot vanilla syrup in a thin, steady stream into the bloomed agar-agar mixture.

Increase the mixer speed to high and whip until the mixture becomes thick, glossy, and at least doubles in volume, 4 to 6 minutes; it should look like soft marshmallow fluff.

Quickly scrape the mixture into the prepared baking dish and smooth the top with a lightly greased spatula.

Dust the top lightly with more sugar free powdered sweetener and let the marshmallows cool uncovered at room temperature until fully set, about 45 to 60 minutes.

Once firm, loosen the slab from the pan, transfer to a cutting board, and cut into 36 squares, dusting all sides with additional powdered sweetener to prevent sticking.