1. Prepare the Fruit Layer:

Preheat your oven to 350°F (175°C). Grease your 11×7-inch baking dish.

In a large bowl, combine the chopped rhubarb, sliced strawberries, ⅔ cup of the sugar, and the cornstarch.

Toss gently until the fruit is evenly coated.

Pour the fruit mixture into the prepared baking dish, spreading it evenly.

2. Cream Butter and Sugar:

In a separate large bowl (preferably using an electric mixer), cream the softened butter and the remaining ⅔ cup sugar until the mixture is light in color and fluffy in texture. This incorporates air for a lighter cake.

3. Add Egg and Flavorings:

Add the room temperature egg to the creamed mixture and beat well until fully incorporated.

Beat in the grated orange zest and vanilla extract until combined.

4. Combine Dry Ingredients:

In a medium bowl, whisk together the all-purpose flour, baking soda, and salt.

5. Alternate Wet and Dry Ingredients:

Add about one-third of the flour mixture to the creamed butter mixture and beat on low speed just until combined.

Add about half of the buttermilk and beat just until combined.

Repeat, alternating flour mixture and buttermilk, ending with the flour mixture. Beat on low speed after each addition only until the ingredient is just incorporated. Do not overmix.

6. Spoon Batter Over Fruit:

Carefully spoon the cake batter evenly over the fruit mixture in the baking dish. Use the back of a spoon or an offset spatula to gently spread it to the edges, covering the fruit.

7. Bake the Cake:

Bake in the preheated oven for 50-55 minutes, or until the cake is golden brown and a toothpick inserted near the center comes out clean.

8. Cool Completely:

Place the baking dish on a wire rack and let the cake cool completely in the dish. This allows the fruit layer to set further and prevents the cake from breaking when served.

9. Serve:

Cut the cooled cake into squares or rectangles.

Serve as is, or topped with whipped cream and a sprinkle of additional orange zest, if desired. Enjoy!