Preheat + Prepare: Set oven to 350F lightly butter a 9x9 square baking pan. l prefer to line my pans with parchment paper to easily lift out the cooled cake, and for easy cleaning.
Cream + Lemon: Stir the fresh squeezed lemon juice into the 40% cream and set aside to thicken.
Butter + Sugar: Cream the soft butter and sugar together until fluffy and light in color.
Egg + Vanilla: Whisk egg and vanilla with a fork.
Zest + Lemon Cream: Add lemon cream mixture and zest to the egg and vanilla
Flour the Fruit: Toss 1 tablespoon of the flour + 1 Tbs sugar with the rhubarb.
Dry Goods: Whisk together dry ingredients - flour, baking powder, and salt.
Add the dry ingredients to the wet ingredients by mixing half of the dry goods, and then half again, until mixed. Batter will be thick.
Fold in Fruit: with a spatula or wooden spoon gently fold in rhubarb. (rhubarb is actually a vegetable, in this recipe I am treating it as a “fruit”)
Spread + Sprinkle: place cake mix into the prepared pan, sprinkle top evenly with sugar. An offset spatula helps with spreading, or push in with the back of a spoon.
Bake 350°F/180°C for about 40-43 minutes in an 8x8 metal pan. You can also use an 8x8 ceramic dish, which takes a bit longer to heat (40-45 minutes). Bake until cake turns golden, and when a toothpick poked in center comes out clean. Let the cake cool sufficiently before cutting.