Preheat the oven to 350°F and grease a square pan.
In a large bowl, whisk together the flour, baking powder, baking soda, and salt
In another bowl, whisk the plant milk, maple syrup, lemon zest and lemon juice, and vanilla until combined. Add canola oil while whisking.
Pour the wet mixture into the large bowl with the flour mixture and gently whisk until just mixed. Do not over-mix.
Pour the cake batter into the pan and shake gently to level the surface.
Top the cake with the rhubarb.
Bake for 50-55 minutes, or until the cake is quite golden and a toothpick inserted into the centre comes out clean. It will be a dark colour due to the maple syrup.
Notes
Zest two small lemons and use the juice or use 1/4 cup of lemon juice