1 Preheat oven 375°F.
2 Lightly grease the insides of a jumbo muffin mold.
3 In a medium sized bowl, mix rhubarb, cornstarch, and granulated sugar.
4 Melt 4 tbsp of butter in small saucepan over med heat, then add brown sugar. Mix until well incorporated and brown sugar is mostly melted.
5 Remove from heat and stir in your vanilla extract (this is your caramel).
6 In a medium bowl mix the all-purpose flour, baking powder, brown sugar, salt, and cinnamon.
7 Using a stand or handheld mixer, beat the softened butter for a minute on medium, then add both granulated and light brown sugar and continue beating on medium until everything is light and fluffy.
8 Scrape down sides as needed, then add in your egg and vanilla extract. Beat another minute on medium and then add ⅓ of your dry ingredient mixture.
9 After that is incorporated, add ½ of your buttermilk, then another ⅓ of your flour mixture, followed by the last half of your buttermilk. Finish with the last ⅓ of the flour mixture and mix until there is no visible dry flour left.
10 Place 1 tbsp of caramel into bottom of each muffin mold. Then pour in ¼ cup of the rhubarb with its juices, then ½ cup of batter.
11 Bake for 30 min or until a toothpick comes out clean. Let cool for 10 min, no longer, then run a small knife around the edge of each muffin to loosen. Use a cooking rack to carefully flip the muffin pan over. Serve warm.
