Preheat the oven to 350 degrees Fahrenheit. In a medium bowl, stir together the flour, baking powder, cardamom, and salt until combined. Set aisde.

In the bowl of a stand mixer fitted with the paddle attachment, beat together the butter and sugar at medium speed until combined, light, and fluffy, about 1 minute. Add the eggs, one at a time, mixing well after each addition. Reduce the mixing speed to low, and alternate between adding the flour mixture and the milk in two increments. Add the plum juice, bourbon, lemon zest, and vanilla extract and mix until incorporated, taking care not to over mix. Fold in the finely chopped fresh plums by hand.

Pour the batter into a well-greased and lightly floured 6-cup capacity cake pan. Bake in the oven until a toothpick inserted into the center of the cake comes out relatively clean (a bit of plum goop is fine, just no wet batter), about 25 to 35 minutes. Remove from the oven and allow to cool for 10 minutes before inverting onto a wire rack and unmolding the cake. Allow to cool to room temperature, then top with the glaze + fruit.

Whisk together the powdered sugar, milk, whiskey, and vanilla extract in a small bowl until combined. Feel free to adjust the quantities to your desired consistency (so if you want the glaze to be thinner, use more milk. If you want it to be thicker, use less milk).