In a small bowl, mix together all-purpose flour, almond flour, brown sugar, and cinnamon.
Add cold cubed butter, and using your fingers or a pastry blender, mix until the mixture forms coarse crumbs.
Place the bowl in the fridge while you prepare the cake batter to keep the butter firm and create a crisp topping.
In a small pot, melt the unsalted butter over medium heat. Continue cooking, swirling the pan occasionally, until the butter turns golden brown and develops a nutty aroma (about 3-4 minutes).
Remove from heat and let it cool slightly.
Preheat your oven to 350°F (175°C).
Line a 9x9 inch baking pan with parchment paper for easy removal.
In a large bowl, whisk together the all-purpose flour, almond flour, baking powder, pinch of salt, cinnamon, and cardamom. Set aside.
In another large bowl, combine the browned butter and granulated sugar together until smooth and creamy.
Add the eggs and egg yolk one at a time, mixing well after each addition. Then, add the vanilla and orange zest.
Gradually add the dry ingredients to the wet ingredients, folding gently with a spatula until just combined. Be careful not to overmix, as this can make the cake dense. Stir in the Greek yogurt (or sour cream), mixing gently until fully combined.
Pour the batter into the prepared pan and smooth the top.
Arrange the halved plums on top of the batter, skin side down, gently pressing them into the batter.
Remove the streusel from the fridge and sprinkle it evenly over the plums.
Bake in the pre-heated oven for 40-50 minutes, or until a toothpick inserted into the center comes out clean or with just a few moist crumbs.
If the streusel starts to brown too quickly, loosely cover the cake with aluminum foil halfway through baking.
Allow the cake to cool in the pan for about 15-20 minutes before transferring it to a wire rack to cool completely.
Notes
Brown the butter carefully - When browning the butter, keep a close eye on it. It can go from golden and nutty to burnt very quickly. Don’t overmix the Batter - once you add the flour mixture, mix just until combined. Arrange the plums cut-side up - Placing the plums cut-side up prevents too much moisture from soaking into the cake.