Italian plums are good to eat, but even better to bake with. They aren't quite as juicy or sweet as their American cousins, but they keep their shape well and their skin adds a gorgeous purple color to tarts and cakes. The brown sugar cake batter and sweet crumb topping are a perfect complement to the not-too-sweet fruit. It is great for breakfast with a cup of tea and/or with ice cream after dinner. This is a cake that also seems to get better the second day, if there is any left.
In a stand mixer fitted with a paddle attachment, cream together the butter and brown sugar, until light and fluffy, about 3 minutes. Add the eggs, one at a time, scraping down the bowl after each. Mix in the zest and vanilla. In a separate bowl whisk together the flour, baking powder and salt. Add the dry ingredients to the butter/sugar mixture and mix on low speed until just combined. Spread half of the batter into the prepared pan.
Place half of the cut plums on the batter, cut-side up. Spread the remaining batter over the plums. Spread the batter into an even layer and top with remaining plums.
To make the topping:
In a bowl, mix together the flour, sugar, salt, cinnamon and melted butter. You may have to mix it with your hands to get all the butter incorporated.
Spread the crumb topping over the plums evenly.
Place the cake on two stacked cookie sheets, to prevent the bottom from getting too brown during the long baking time. Bake for about 1 hour 15 minutes or until a tester come out clean. Cool on a rack.
Serve it slightly warm or at room temperature.