Use a sharp knife to carefully split each vanilla bean lengthwise, revealing tiny black seeds along the interior.
Scrape out the vanilla seeds with the back of the knife and place them in a blender jug or a large mixing bowl.
Add the two vanilla bean pods to the blender along with the ice cubes, heavy cream, and sugar if using.
Blend on high speed until the mixture is smooth and icy, with the vanilla seeds evenly dispersed and the texture creamy yet frozen.
Taste the mixture and adjust sweetness or add more ice if you prefer a colder, thicker consistency. Briefly pulse to combine.
Pour the frozen vanilla slushie into chilled glasses, garnish with a vanilla bean or a sprinkle of vanilla seeds if desired.