Let’s bake this elegant single-layer cake:

1. Prepare Pan and Preheat Oven:

• Preheat the oven to 350°F (180°C). Line an 8×8 inch baking pan with parchment paper, leaving an overhang on two sides for easy lifting. Lightly grease the parchment and pan sides. Set aside.  

2. Combine Sugar, Zest, and Dry Ingredients:

• In a stand mixing bowl (or large bowl), combine the granulated sugar and fresh lemon zest. Rub them together with your fingers until the sugar is fragrant and slightly moist.
• Add the cake flour, baking powder, and fine sea salt to the sugar mixture. Whisk briefly or mix on low speed (paddle attachment) until just combined.

3. Add Butter (Reverse Creaming Style):

• Add the cubed room temperature butter to the dry ingredients. Mix on low speed until the mixture resembles coarse sand and all the butter is broken down into small pieces coated with flour.

4. Combine Wet Ingredients:

• In a separate medium bowl or large measuring cup, whisk together the room temperature whole milk, room temperature sour cream, canola oil, room temperature egg, and vanilla extract.

5. Add Wet to Dry:

• With the mixer still on low speed, slowly stream the wet ingredients into the dry ingredients/butter mixture. Mix just until combined. Do not overmix. Scrape the bowl once or twice.

6. Bake the Cake:

• Pour the batter into the prepared 8×8 inch baking pan and spread it evenly.
• Bake for 23-26 minutes, or until a toothpick inserted into the center comes out clean or with only a few moist crumbs attached.
• Let the cake cool completely in the pan on a wire rack.

7. Make the Mascarpone Frosting:

• In a clean, chilled mixing bowl (preferably stand mixer with whisk attachment, or use hand mixer), beat the cold mascarpone cheese briefly with the powdered sugar on low speed until just combined.
• Add the cold heavy whipping cream, vanilla extract, and lemon zest. Whisk on low speed to combine, then increase the speed to medium-high.
• Beat just until stiff peaks form. Watch carefully and do not overmix, or the frosting can become clumpy, grainy, or curdled. It should be light and hold its shape.

8. Prepare the Berry Topping:

• In a medium bowl, gently combine the fresh blackberries, chopped strawberries, raspberries, and the tablespoon of jam. Toss lightly to coat the berries.

9. Assemble the Cake:

• Once the cake is completely cool, spread the prepared mascarpone frosting evenly over the top of the cake (still in the pan or removed using the parchment sling).
• Spoon the mixed berry topping evenly over the mascarpone frosting.

10. Serve:

• Serve immediately, or chill briefly before serving if desired.

This assembly creates a beautiful single-layer presentation