Preheat oven to 350 degrees F (175 degrees C).
Butter/oil a baking tin (we used a round tin) and cut out a sheet of parchment paper in a circle to cover the bottom.
Mix together the chopped fresh rhubarb and 1.5 tbsp of granulated sugar. Let this marinate while you mix the rest of the cake batter.
Beat eggs and granulated sugar together until light in color and doubled or tripled in size. Use a stand mixer with a whisk attachment, or an electric hand mixer - it will be more difficult to whisk by hand, but it can be done!
Meanwhile, mix together dry ingredients (flour, baking powder, salt) in a separate bowl.
Mix together vanilla extract, milk, and oil in a liquid measuring cup to make it easier to pour.
Alternate adding in wet ingredients and dry ingredients, adding half of each at a time.
Fold into the mixture with a rubber spatula or mix on low speed in the stand mixer.
Add most of the chopped rhubarb and sugar to the cake batter, reserving a small amount that you will add on top. Fold the batter a few times to make sure the rhubarb is evenly distributed, but try not to overmix at this point.
Pour the batter into your baking tin.
Sprinkle the remaining rhubarb over the top of the cake. You can also sprinkle some demarara sugar for a bit of crunch, but that is totally optional!
Bake for 40-45 mins, or until a cake tester emerges clean from the center of the cake. (Baking time will depend on your baking tin and oven. A shallower, larger cake tin will take less time, while a smaller but deeper tin may take longer. Glass tins also tend to take longer than dark, nonstick ones. Ovens range, so keep an eye on the cake!)
Remove from cake tin after about 5 minutes and let cool (we let ours cool on a wire rack). Slice and enjoy!