For the bundt cake: Preheat the oven to 350 degrees F. Grease a bundt pan well and set aside.
In a bowl, mix together the flour, baking powder, salt and powdered vanilla and set aside. In a large measuring cup, mix together the milk and mandarinello and set aside.
In the bowl of a stand mixer fitted with a paddle attachment, cream together the butter and granulated sugar until well combined. Add the eggs and orange zest and mix until you have a creamy-looking mixture.
Add the flour mixture and, with the motor running on medium speed, drizzle in the milk and mandarinello mixture and mix until the batter is smooth. Pour into your prepared pan and bake until fully cooked through, around 1 hour and 10 minutes. Allow to cool completely, then invert the cake onto a platter.
For the mascarpone cream: In a bowl with a whisk, blend the mascarpone with the granulated sugar, mandarinello and vanilla paste until light and creamy. In a separate bowl, whip the cream to soft peaks.
Using a large rubber spatula, add a big dollop of whipped cream to the mascarpone mixture and stir together to lighten. Then add the rest of the whipped cream and gently fold into the mascarpone to make a light, fluffy mixture.
For the blackberry compote: Combine the sugar, lemon zest and juice in a medium saucepan over medium-high heat. Add the blackberries and cook until the berries release their juices, 6 to 7 minutes depending on how ripe the berries are. Continue to cook until the sauce has thickened slightly but the berries are still somewhat whole. Serve immediately, warm, or cool to room temperature about 10 minutes, then refrigerate, covered, to serve cold.
Dust the cake generously with confectioners' sugar and serve slices with some blackberry compote and a dollop of mascarpone cream.
Italian powdered vanilla can easily be replaced by vanilla extract. I like to use it because it¿s what my Nonna uses-¿ so then my cake tastes just like hers. But feel free to use whatever you like ¿ it will still be phenomenal.