How to Make It
Preheat the oven to 325°; butter a 10-inch cast-iron skillet. In a bowl, toss the plums with 2 tablespoons of the sugar and the lemon juice.
In a food processor, pulse the flour with the baking powder, salt, cinnamon, cardamom and the remaining 1/2 cup of sugar. Add the butter and process until finely chopped. Add the eggs and vanilla and process until incorporated. With the processor on, gradually add the milk in a thin stream and process until the batter is smooth. Add the batter to the plums, gently folding them in.
Transfer the batter to the prepared skillet and smooth the surface. Bake the cake for about 1 hour, until the top is golden and a toothpick inserted in the center comes out clean. Let cool in the pan.
In a medium bowl, beat the heavy cream with the honey at medium-high speed until softly whipped. Beat in the mascarpone at medium-low speed.
In a food processor, pulse the Hazelnut Brittle until coarsely chopped.
Cut the cake into wedges and transfer to plates. Spoon a dollop of the honey mascarpone on top, sprinkle with the chopped nut brittle and serve.