Preheat the oven to 175C(350F) and grease a 1lb loaf tin with oil spray or a little unsalted butter.
Make the streusel first by placing the flour and butter in a large bowl and rubbing together with your fingertips until you get breadcrumbs (I like to have a mix of small breadcrumbs and large lumps). Stir in the sugar and salt. Put to one side.
Now make the cake. Mix the egg and sugar in a large bowl. Add the oil and stir. Add the vanilla extract, almond extract, ground almonds and Greek yogurt and stir again.
Sift in the flour, baking powder and baking soda. Fold together, using a spatula, until combined. Pour half of the mixture into the prepared baking tin. Layer half of the plum slices on top of the mixture, and sprinkle on half of the streusel mixture.
Top with the remaining cake mix and layer on the remaining plum slices in two lines. Arrange the remaining streusel mixture around the plums (it's fine if it covers the plums a little, but try not to hide them completely).
Place in the oven to cook for 55-65 minutes (until and inserted skewer comes out clean). Take out of the oven and leave to cool in the tin for 10 minutes then remove from tin and allow to cool further on a cooling rack.
Whilst the cake is cooling, make the drizzle. Place the confectioners' sugar in a bowl and add the almond extract and half the water. Whisk together until thick and smooth. Add a little more water if needed until you get thick drizzling consistency.