Preheat oven to 375 degrees Fahrenheit.
Grease a 10-inch baking dish with butter or olive oil and lightly dust with flour. Set aside.
Cream together pieces of butter and sugar on low speed in a large bowl until creamy and smooth (with a handheld mixer).
One by one add eggs, beating on low speed again after each addition.
Stir in vanilla extract.
In a separate bowl combine flour, baking powder, salt, and cinnamon. In batches add the flour mixture to wet ingredients and combine, scraping the bottom of the bowl and sides with a wooden spoon or spatula. Don’t overmix the flour-sugar mixture on increasing the speed.
In a small bowl combine 1 teaspoon flour and rhubarb slices. Toss the rhubarb and shake off any excess flour (a small strainer will help with this).
Fold the prepared rhubarb into the cake batter.
Pour the batter into the prepared cake dish and even out the top with a pastry spatula. Decorate with chopped rhubarb slices gently pressing them into the batter halfway through. Dust with brown sugar. (For decoration: I cut rhubarb stalks into 6 smaller long pieces (about 4 inches) + 4 pieces (1 inch)).
Bake for 40-45 minutes or until the top of the cake is golden brown with a light crisp exterior. Test the center of the cake with a toothpick, if it comes out clean, your rhubarb cake is done. Let the cake rest for 5-10 minutes before slicing. Dust with optional confectioners’ sugar. Enjoy!