• Preheat oven to 350 degrees Fahrenheit (177 degrees Celsius). Grease a 9 inch (23 cm) springform cake pan, set aside.
• Halve, pit and slice the plums into 8 pieces.

Make the topping:

• In a small bowl, mix together the almonds, flour, brown sugar, cinnamon, cardamom and melted butter until combined. Set aside.

Make the cake:

• In a medium bowl, sift together the flour, baking powder, baking soda, ground cinnamon and salt. Set aside.
• In a large bowl, using a stand mixer fitted with a paddle attachment or a hand-held mixer, beat together the butter and sugar until light and fluffy (about 3 minutes).
• Add the eggs to the butter mixture one at a time, until well combined, scraping down the sides of the bowl with a spatula as needed. Add vanilla extract. Stir in half of the flour mixture, then the buttermilk, then the rest of the flour mixture until just combined. Be careful not to overmix.
• Pour the batter into the prepared pan and spread evenly with a spatula. Place the plum slices in an even layer over the top, then spread the almond topping over the plums.
• Bake for 55 minutes to 1 hour, until the centre feels just set (a toothpick inserted into the centre should be clean). Check the cake in the last 10 minutes of baking, if the top is browning too much, cover with aluminium foil. Once done, cool cake completely on a wire rack and remove the sides of the springform pan.

Make the toffee sauce:

• In a small saucepan, mix the butter, brown sugar, cream and vanilla and bring to a gentle boil, stirring continuously. Reduce heat and cook at a low boil for 1 minute. Remove from the heat and spoon the sauce over the cake when ready to serve.