Position rack in centre of oven and preheat oven to 175°C (155°C fan-forced). Generously brush a 22cm x 12cm loaf pan with softened butter and line the base with baking paper.
In a medium saucepan over high heat, bring raisins and ½ cup (125ml) water to boil. Reduce heat to medium. Simmer for 4 mins or until raisins are tender and liquid has evaporated. Stir in bicarbonate of soda. Remove from heat. Cool.
In a small bowl, stir flour, baking powder, cinnamon and ¼ tsp salt to blend. In a large bowl, using an electric hand mixer, beat 90g butter, sugar and 1½ tsp orange zest on medium-high speed, scraping down side as needed, for 2 mins or until blended. Add eggs and vanilla and beat for 1 min or until batter is light and airy. Reduce speed to low. Add ¾ cup (185ml) orange juice in 2 batches, alternating with flour mixture. Beat on medium-high speed for 1 min (batter may appear split but this will not affect finished cake). Fold in raisins, carrots and walnuts.
Pour batter into prepared pan. Wrap pan tightly with plastic wrap. Place in a 33cm x 22cm baking dish. Add enough boiling water to come halfway up sides of pan. Cover baking dish tightly with foil. Carefully transfer dish to oven. Bake for 1 hour 45 mins or until cake springs back when gently pressed and a toothpick inserted into centre comes out clean. Remove foil and plastic wrap. Transfer pan to a wire rack for 10 mins to cool. Turn onto a serving platter. Slice.
To make mascarpone cream, in a medium bowl, using an electric hand mixer on medium speed, beat cream, mascarpone and sugar until soft peaks form. Serve with the cake.