Position oven rack in the middle position and preheat oven to 400 degrees. Line a standard 12 cup muffin pan with paper or foil liners.
Whisk flour, baking powder, baking soda, and salt in a medium bowl.
In a separate large bowl, whisk together eggs, buttermilk, sugar, oil, and vanilla. Add flour mixture and mix with light strokes until the dry ingredients are just moistened. Don’t over mix; batter should not be smooth.
Add grapes and combine. Divide batter among the muffin cups and bake until a toothpick inserted in one or two of the muffins come out clean and the tops are golden brown, about 12-15 minutes. Cool on a baking rack.
While the muffins are baking, mix together peanut butter and honey in a small shallow bowl. Add one to two TBS warm water so that mixture thins out to a smooth glaze.
As soon as the muffins are cooled for a few minutes, dip them one at a time in the glaze. Set on a rack to cool.