For the cake batter: Preheat the oven to 350 degrees F; line a cupcake pan with 12 cupcake liners.
Sift together the flour, granulated sugar, cardamom, baking powder, baking soda and salt in a large bowl. Add the buttermilk, oil, vanilla extract and eggs and mix until smooth and completely combined.
Pour the batter into the cupcake liners and bake until golden brown and risen, 8 to 10 minutes. Let cool completely before frosting.
For the rhubarb compote: In a medium saucepan, combine the rhubarb, granulated sugar and 1/4 cup water. Bring to a simmer over medium heat and cook until the water is absorbed and the rhubarb softens. Set aside to cool.
For the cardamom frosting: In a medium mixing bowl using an electric mixer, cream the butter. Add the confectioners' sugar 1/4 cup at a time and mix until completely smooth. Mix in the cardamom and vanilla extract.
To assemble: Remove the centers of each cupcake using a paring knife. Fill each with some of the rhubarb compote and frost with the cardamom frosting. Serve immediately, or store in an airtight container for up to 2 days.
This recipe was created by a contestant during a cooking competition. It has not been tested for home use.