Preheat your oven to 180°C (350°F). Grease your 9-inch round cake pan with butter or line it with parchment paper for easy removal.
In a large mixing bowl, combine the softened unsalted butter and castor sugar. Using an electric mixer or whisk, beat them together until the mixture is light and fluffy. This should take about 3-5 minutes.
Add the sour cream (or yogurt) to the butter and sugar mixture, and mix until well combined. Then, gradually add the lightly beaten eggs, mixing well after each addition. Stir in the vanilla extract and lemon (or orange) zest.
In a separate bowl, whisk together the all-purpose flour and baking powder. Gradually add this dry mixture to the wet ingredients, folding gently with a spatula until just combined. Be careful not to overmix!
Gently fold in your choice of fruits—this can be a colorful mix of strawberries, blueberries, or diced peaches. If you’re using fruit that’s very juicy, consider tossing them with sugar beforehand to enhance their flavor.
Pour the batter into the prepared cake pan, smoothing the top with a spatula. Bake in the preheated oven for 40-50 minutes, or until a toothpick inserted into the center comes out clean.
Once baked, remove the cake from the oven and let it cool in the pan for about 10 minutes. Then, transfer it to a wire rack to cool completely. Serve as is, or dust with powdered sugar for a touch of elegance!