Preheat the oven to 350℉. Line an 8-inch round pan with parchment paper or grease and dust it with flour.

With a fine mesh strainer, sift flour, baking powder, and salt into a medium bowl and whisk together.

In a stand mixer, cream the butter and sugar for 5 minutes or until it's fluffy and light. Scrape down the bowl with a spatula.

Add the eggs one at a time to the butter mixture, beating well after each addition. Mix in vanilla extract. Scrape down the bowl again.

With the mixer on the lowest speed, add the flour mixture and milk alternatively, beginning with one-third of the flour mixture and half of the milk; repeat, then finish with the flour mixture. Scrape down the bowl and finish mixing the batter by hand.

Pour the batter into the cake pan and smooth the top. Bake on the middle rack for 23-26 minutes. A toothpick inserted should come out clean of batter, but a few crumbs are okay. Remove the cake and let it cool in the pan for 15 minutes before transferring it to the refrigerator for 30 minutes.

Whisk together the flour, brown sugar, lemon zest, and salt. Work in the butter, mixing until coarse crumbs form.

Sprinkle over a small baking sheet and bake for 7 minutes in a 350℉ oven. Allow it to cool and then break it into small pieces.

In a medium-sized mixing bowl, beat the mascarpone cheese until soft. Add the heavy cream, sugar, and vanilla and beat until stiff peaks form.

Stir in the lemon juice and refrigerate until ready to assemble the cake.

Cut the cake horizontally in half. Place one layer of the cake on a cake plate. Add two-thirds of the filling, then top with the second layer.

Cover the top and sides with the remaining filling and top with the streusel topping. Dust the cake with powdered sugar, and enjoy!