Preheat your oven to 350 degrees, and generously grease an 8 inch cake pan, either a decorative one, or a standard cake pan.
In a large bowl, whisk together the sugar, melted butter, canola oil, and grapefruit zest until combined. Stir in the egg until just combined. In a medium bowl, stir together the flour, almond flour, salt, ground cardamom, baking powder, and baking soda. In a small bowl, stir together the whole milk and the grapefruit juice until combined.
Add the flour mixture and the milk mixture to the sugar mixture in alternating batches, stirring well between each addition, until all of the dry and wet ingredients have been stirred in. Pour the batter into your prepared cake pan. Bake for 25-28 minutes, or until a cake tester comes out clean. Cool the cake for 10 minutes in the pan before turning it out onto a cooling rack to finish cooling completely.
In a medium bowl, using an electric mixer, beat the mascarpone cheese and powdered sugar together for 1-2 minutes, or until fluffy and smooth. Add in the heavy cream, cardamom, and vanilla extract, and beat for 3-4 minutes, or until stiff peaks have formed and the mixture is very fluffy. Pile it on top of your cooled cake, and sprinkle the sliced almonds on top. Slice and enjoy!