Heat oven to 350°F (180°C). Grease and flour a 9x13-inch baking pan or a 10-inch round cake pan.
In a small bowl, combine flour, sugar, and cold butter. Use a pastry cutter, fork, or food processor to blend until crumbly. Keep chilled until ready to use.
In a medium bowl, whisk together flour, baking powder, salt, and cinnamon (if using).
In a large mixing bowl, beat eggs with sugar until smooth. Add melted coconut oil and mix until combined.
Stir the baking soda into the yogurt until it dissolves and foams slightly. Add to the egg mixture and stir well.
Gently fold the dry ingredients into the wet mixture until just combined. Do not overmix.
Fold the diced plums into the batter. Spread evenly in the prepared pan.
Sprinkle the streusel mixture evenly over the batter. Bake for 40–45 minutes, or until a toothpick inserted in the center comes out clean.
Let the cake cool completely before slicing. Store covered in the refrigerator for up to 3 days.