Preheat the oven to 350 degrees Fahrenheit (175 degrees Celsius). Line a 12-cup muffin tin with paper liners or lightly grease it.
In a large bowl, whisk together the flour, sugar, baking powder, salt, and cinnamon (if using).
In a separate bowl, whisk together the plant-based milk, vanilla extract, oil, and vinegar.
Pour the wet ingredients into the dry ingredients and stir until just combined—do not overmix.
Gently fold in the chopped grapes until evenly distributed.
Divide the batter evenly among the muffin cups, filling each about three-quarters full.
Bake for 25 minutes, or until a toothpick inserted in the center comes out clean.
Let the muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.
Notes
Don’t use too much flour: Don’t scoop it. Instead, fluff it in its bag with a fork, spoon it into a measuring cup, and level with a knife to avoid dry, dense muffins.
Don’t over-mix: To avoid dense, gummy muffins, mix the batter until only just incorporated – a few small lumps are fine.
Use seedless grapes: This makes a BIG difference to texture and enjoyment. Otherwise, the seeds will need to be removed first.