Preheat the oven to 350°F. Grease a 9-inch round cake pan or springform pan.
In a large mixing bowl, whisk the flour, baking powder, salt, cinnamon, and nutmeg. Set aside.
In a stand mixer or in a large mixing bowl with an electric hand mixer, beat the softened butter and granulated sugar for a few minutes, until it is creamy and fluffy.
Add the eggs and almond extract and beat again until it’s well-combined.
Add the Greek yogurt and mix with a silicone spatula until everything is well-combined.
Add the dry ingredients to the wet ingredients, and gently mix with a large silicone spatula just until there's no dry flour visible.
Spread the batter in the greased cake pan and smooth the top of the batter with a spatula. Set it in the fridge for 15 minutes to firm up.
Prepare the streusel by mixing the melted butter, flour, brown sugar, and cinnamon in a small bowl until it looks clumpy.
Slice the plums in half and remove the pits. Gently press the plums cut-side-up onto the top of the chilled cake batter (don’t bury them down too deeply). The plums will sit pretty high on top of the cake, but that gives plenty of room for the streusel & for the cake to rise up around the plums.
Sprinkle the streusel evenly on top of the plums and cake.
Bake for 55 to 70 minutes, until a toothpick comes out clean from the center of the cake. Slice & enjoy!