Preheat oven to 350°F (175°C) and grease a 9-inch cake pan.
Cream softened butter until light and creamy, then mix in caster sugar.
In a separate bowl, combine all-purpose flour and baking powder.
In another bowl, whisk together 1 large egg, 1 teaspoon vanilla extract, and 1/2 teaspoon kosher salt.
Alternate adding flour and milk to the mixture, starting and finishing with flour.
Gradually add the dry ingredients to the butter mixture, alternating with the wet ingredients, until well combined.
Fold in fresh strawberries and peaches.
Pour the batter into the prepared cake pan and smooth the top with a spatula.
Bake for 30-35 minutes or until a toothpick inserted into the center comes out clean.
Optional: Melt apricot jam and brush it over the fruit for a shiny glaze.