Can I use frozen rhubarb?

Yes, but thaw it first and pat it dry. Frozen rhubarb releases more liquid, so drying helps the batter stay thick.

Do I have to broil the glaze?

No. It will still taste good, just more like a soft icing. Broiling is what gives that sweet crackly finish.

Most likely they baked too long or there was a bit too much flour. Next time, pull them earlier and measure flour lightly.

I would not. The baking powder starts working right away, so the batter is best baked soon after mixing.

If it is thin, add more powdered sugar. If it is too thick, add milk or lemon juice a few drops at a time until it spreads.

Scale

Cook Mode Prevent your screen from going dark

Preheat oven to 400°F and line a sheet pan with parchment paper.
Mix the dry ingredients (flour, baking powder, sugar, and salt) in one bowl.
Whisk the egg, milk, vanilla, and melted butter in another bowl.
Pour the wet mixture into the dry mixture and stir gently until barely combined.
Fold in the diced rhubarb until evenly distributed.
Scoop batter onto the prepared sheet pan using a cookie scoop or two spoons, leaving space between each mound.
Bake for 12 to 16 minutes until lightly golden and set.
Prepare the glaze by mixing powdered sugar with a tiny splash of milk or lemon juice, vanilla, and a pinch of salt until thick.
Glaze the warm fritters with the mixture, brushing it on evenly.
Broil briefly for 30 to 90 seconds to set the glaze but watch closely to avoid burning.

Serve warm with extra glaze, yogurt, strawberries, or ice cream. Store leftovers in a container with slight ventilation.