A moist coffee cake with a hint of lemon and the most delicious nutty streusel to top.
Preheat the oven to 350F and grease a 9" baking pan (square or round). Prepare the streusel by mixing together all of the ingredients; set aside.
In a medium bowl, whisk together the flour, baking soda, baking powder and salt.In a large bowl, beat the butter and sugar together at high speed until light. Add the egg and beat until fluffy.
Add the yogurt, vanilla, lemon juice and lemon zest and beat until smooth. Add the flour mixture and beat on low speed until just mixed, scraping the sides of the bowl.
Transfer the batter into the prepared pan and spread to smooth with a rubber spatula. Place the plum halves cut-side-down arranged on top of the batter in your pan.
Sprinkle the streusel evenly over the top and bake for 30-35 minutes or until a toothpick inserted in the centre comes out clean. Serve as-is or with a scoop of vanilla ice cream.