Butter a 9-inch round springform pan and line the bottom with parchment paper. Preheat the oven to 350°F.

In the bowl of a stand mixer fitted with the paddle attachment, combine butter, white sugar and ¼ cup brown sugar. Beat at medium speed until light and creamy.

Crack in the egg and add the vanilla. Beat again until very well combined.

Sift the flour and baking soda into a separate bowl. Whisk in the salt and nutmeg. Measure out 2 tablespoons of this flour mixture and toss with the rhubarb and carrots in another bowl.

With the mixer on Low, add the dry ingredients to the creamed butter in three parts, alternating the buttermilk with the flour, finishing with the flour. Mix well with each addition.

Add the rhubarb & carrots to the cake batter and gently stir to combine. Scrape the batter into the prepared cake pan and smooth the top.

Mix together the remaining ¼ cup of brown sugar with the cinnamon. Sprinkle over the cake batter. Run a butter knife through the cake, making a few figure eights to swirl the cinnamon into the batter.

Place the cake on a middle rack in the centre of the oven and bake until a toothpick inserted in the middle comes out clean. About 1 hour or 1 hour and 10 minutes.

Cool the cake for 10 minutes on a wire rack, then loosen around the edges and remove the ring. When cool, frost generously with Whipped Mascarpone Frosting, then slice and serve.

In the bowl of a stand mixer fitted with the whisk, combine all of the ingredients for the frosting. Whip on high until the cream holds soft peak.

Combine the sugar and water in a small pot. Warm over medium heat until the sugar has completely dissolved. Set aside to cool.

Wash and trim the rhubarb, cut in half crosswise and then lengthwise. Peel firmly into long, even strips. Submerge the rhubarb strips in cooled sugar syrup and transfer them to a parchment lined baking sheet.

Bake until the strips are dried out and fairly crisp. Start checking after 35 minutes.

Once they are keeping their shape, transfer them to a paper lined plate and use.