In a small bowl, toss the finely diced rhubarb & apple with 2 Tbsp sugar & 1 tsp cinnamon. let sit for 10 minutes to draw out a little moisture & soften the rhubarb’s sharp edge.
Fritters
Preheat oven to 400 F. Line a large baking sheet with parchment paper.
In a large bowl, whisk together flour, white & brown sugar, baking powder, salt , cinnamon & cardamom.
In a separate bowl, whisk together the milk, eggs, melted butter & vanilla.
Pour wet ingredients into the dry ingredients. Stir gently with a spatula until just combined (do not overmix). Gently fold in the spiced rhubarb/apple chunks along with any juices left in the bowl.
Scoop heaping 1/4-cup portions of the batter onto prepared baking sheet, leaving about 2 inches of space between them.
Bake 15-18 minutes or until the tops are golden brown & a toothpick inserted into the center comes out clean.
Glaze
If you are using the glaze, whisk the powdered sugar, milk (or apple cider) & vanilla in a small shallow bowl until smooth. Let fritters cool on a wire rack for 5 minutes, then dip the tops of the warm fritters into the glaze. Let the glaze set for a few minutes before serving.