For the simple one-bowl VANILLA BATTER: Preheat the oven to 360°F/180°C. Place all the dry ingredients for the cake base in a mixing bowl and combine, add the wet ingredients and whisk until it's a smooth batter.
For the STREUSEL: Put the cold butter, all-purpose flour, cinnamon powder, and granulated sugar in a bowl, mix and knead with your hands until crumbly.
ASSEMBLE and BAKE: Oil the baking pan lightly and pour in the cake batter, place the plum halves on the cake batter and press them down a bit. Sprinkle the crumble on top. Bake it in the oven for about about 25 minutes. When the crumble is nice and golden, your cake should be ready. Test with a wooden stick, if it is baked through.