Preheat your oven to 350°F (175°C). In a large mixing bowl, cream together unsalted butter and granulated sugar until light and fluffy, about 3–4 minutes. Beat in eggs one at a time. Add orange juice and zest, mixing until combined. Gradually add flour, baking powder, and salt. The batter should be smooth and slightly thick.

Gently fold in fresh raspberries into the batter using a spatula. Be careful not to overmix. Pour the batter into a greased 9-inch round cake pan and bake for 25–30 minutes or until a toothpick inserted comes out clean.

Remove the cake from the oven and let it cool in the pan for about 10 minutes. Then, gently run a knife along the edges and invert the cake onto a wire rack, allowing it to cool completely.

In a clean mixing bowl, combine mascarpone cheese, powdered sugar, and vanilla extract. Beat on medium speed until creamy. Gradually pour in heavy cream and continue to mix until the frosting holds stiff peaks.

Once the cake has cooled completely, place it on a serving plate or cake stand. Spread an even layer of mascarpone frosting on the top and sides.

Arrange fresh raspberries and thin slices of orange on top as a garnish. Serve immediately or refrigerate until ready.