Preheat the oven to 350°F (180°C). Line an 8-inch cake pan with parchment paper and lightly grease the sides.
In a mixing bowl, combine the sifted flour, baking powder, and salt. Whisk together to evenly distribute.
1 1/2 cups all-purpose flour
In a large bowl, beat together the eggs, sugar, and vanilla extract using an electric mixer or whisk until the mixture is pale and slightly thickened, about 3 minutes.
1 1/2 cups all-purpose flour
Add the melted butter and mascarpone cheese to the egg mixture. Continue mixing until smooth and well incorporated, about 2 minutes.
1 1/2 cups all-purpose flour
Gently fold in the sour cream, followed by the dry ingredients. Mix just until combined, avoiding overmixing to keep the batter light.
1 1/2 cups all-purpose flour
Pour the batter into the prepared cake pan, smoothing the top with a spatula. Tap the pan lightly on the counter to remove any air bubbles.
Bake in the preheated oven for 40-45 minutes, until the top is golden brown and a toothpick inserted into the center comes out clean. The cake should be slightly domed and firm to the touch.
Remove the cake from the oven and let it cool in the pan for 10 minutes. Then, transfer to a wire rack and cool completely before serving.