Preheat oven to 350˚F. Grease a 1/4 sheet pan (9"x13"). Cut parchment paper to fit into sheet pan with some overhang and press into the greased pan then flip it so both sides of parchment get oiled.

Cut and slice plums into about 35 thin slices.

Whisk together flours, sugar, baking powder and soda, salt and lemon zest in a bowl.

Whisk together oil, milk, maple syrup, vinegar and vanilla extract in a separate bowl.

Add wet mixture to dry and gently stir to combine, about 1 minute. Don't overmix.

Pour batter into prepared pan and spread into a even layer.

Arrange plum slices on top of batter.

Bake for about 30 minutes until cake is golden brown and an inserted toothpick some out clean.

Cool in pan for about 10 minutes. Then carefully lift with help of parchment and transfer cake to a cooling rack to cool completely.

Before serving, dust cake with powdered sugar. This will give a sheen to the plums, which may look dry after baking.

Cut and serve. Cover leftovers and store at room temp for up to several days.