Preheat oven to 350F/175C. Grease an 8½ x 4¼-inch loaf pan.
In a large bowl sift together flour, baking powder, and salt. Set aside.
In a mixer bowl fitted with the whisk attachment, beat together eggs and powdered sugar on high speed for 5 minutes, until pale and creamy. Reduce speed to low and slowly add butter followed by mascarpone, lemon zest and juice, and vanilla. Add flour mixture and mix slowly just until combined.
Pour batter into prepared loaf pan. Bake for 40-50 minutes or until a toothpick inserted into the center of the cake comes out clean. Allow cake to cool completely. Sprinkle with powdered sugar if you like.
Store cake in an airtight container in the refrigerator for 4-5 days. Cake is best served at room temperature.