Grease and line a 24cm square cake tin with baking paper.
Add sugar, eggs and vanilla extract to a bowl. Beat with an electric beater for approximately 3-4 minutes until light and fluffy.
Add sifted flour, almond meal, baking powder and lemon zest to a large bowl. Add the egg mixture and gently mix with a wooden spoon until just combined.
Add the melted butter and mix gently until butter is incorporated (do not overmix).
Spoon batter into the prepared tin and smooth the top.
Arrange plum slices evenly over the cake batter.
Prepare the crumble by mixing all ingredients for it in a small bowl. Follow the order of ingredients as follows to ensure that your crumble is crunchy after baking - sugar, butter, cinnamon followed by nuts and flour. Mix only until just combined, do not overmix. Drop small teaspoons full of the crumble over the cake batter and plums.
Bake the cake in the preheated oven for approximately 45 minutes until the crumble is browning and the cake is golden and risen.
Turn off the oven and allow the cake to sit in the. Warm oven for a further 3-5 minutes.
Remove cake from the oven and cool completely in the tin.
Notes
Storage - Stone fruit like plum, peach, nectarine, mango and fresh berries are more likely to spoil rather quickly at room temperature. This cake is best eaten within the first 1-2 days. It can be stored in a cool corner of your kitchen if the weather is cool and crisp. Otherwise, the fridge is the best place to store it. Keeps well in the fridge for up to 4 days.