1. Preheat oven to 350°F/180°C and grease a 9-inch round cake pan. For an easy release of the cake later on, line the bottom of the pan with parchment paper, and use a springform pan if you have one.

   Note: wrap your springform pan with 2 layers of aluminum foil first to catch the juices that will leak out during the baking process.

Prepare the rhubarb & mascarpone mixtures:

1. Combine the sliced rhubarb with corn starch and granulated sugar. Stir and set aside for several minutes. The rhubarb will start to soften and soak in its natural juices (called maceration).

2. Mix together the mascarpone cheese with the juice of one lemon. It will slightly curdle, this is good. Set aside.

Make the caramel:

1. In a small, deep, heavy-bottomed saucepan place butter and sugar. Cook over medium heat until butter is melted, sugar is dissolved, and mixture is bubbling, about 3 minutes. Don’t worry if the mixture Is a bit gritty and not smooth. It would all melt in the oven. Pour into prepared pan and spread in the pan until bottom is evenly covered with the caramel.

2. Place the rhubarb slices flat in the pan in a random or desired pattern, covering most of the caramel layer. Don't add any juices from the bowl of rhubarb. Set aside.

Make the cake:

1. In a medium bowl whisk together flour, baking powder, baking soda, salt, and cardamom. Set aside.

2. In a mixer fitted with the paddle attachment, beat together butter and sugar on medium speed for 3-5 minutes until fluffy and pale in color. Scrape down the sides and bottom of the bowl as necessary.

3. Add the eggs, one at a time, beating well after each addition. Beat in vanilla extract and lemon zest. On low speed, beat in the flour mixture in 3 additions, alternating it with the mascarpone in 2 additions, starting and ending with the flour, just until the batter comes together. Do not over mix.

4. Scoop batter carefully over rhubarb (I use an ice cream scoop) and smooth the top. Bake for 60-65 minutes, or until a skewer inserted into the center of the cake comes out clean. Allow cake to cool in pan on a wire rack for 15-20 minutes, then run a knife along the edges of the pan and invert onto a plate (do not spring the springform pan). If you used parchment paper, gently remove it. Allow cake to cool completely on the plate.