Preheat the oven to 350℉. Line a 9x9 pan with parchment paper and set aside. Chop the strawberries and rhubarb into small pieces (about 1-2 cm thickness) and set aside to use later.
1 cup fresh rhubarb, 1 cup fresh strawberries
In a medium-sized bowl, mix together the dry ingredients. Set aside.
In a large bowl, add the softened butter and white sugar. Cream together on high using an electric mixer for 2-3 minutes. The mixture should be pale in colour and fluffy. Scrap the sides of the bowl as needed.
1/2 cups unsalted butter, 3/4 cups white sugar
Add in the yogurt, egg, yolk, and vanilla. Continue to mix together on high, for another 2-3 minutes until combined. Scrap the bowl again when needed. The mixture can be curdled and that's totally normal.
1 egg, 1 egg yolk, 1/2 cup yogurt, 1 tbsp vanilla
Add in the dry ingredients. With a spatula, gently fold them together until a soft cake batter forms. Don't over-mix!
Into the bowl with the rhubarb and strawberries, add 1 tbsp of flour. Gently toss this around until the flour is evenly coated around them. This will prevent them from sinking to the bottom of the pan as they bake.
Add the strawberries and rhubarb into the cake batter and mix a few times until evenly distributed. Don't over-mix!
Transfer the batter into the prepared pan and spread evenly. Sprinkle a bit of white sugar on top of the cake (optional, but recommended).
Bake the cake for 35-45 minutes. The cake is ready when a toothpick inserted comes out clean. Allow it to cool, slice, and enjoy.
Notes
Buttermilk* You can make buttermilk: Just mix together 1/2 cup of room temperature milk with 1 tbsp of lemon juice or vinegar. Allow it to sit for 10 minutes to activate and then you can use it.