a piping bag fitted with a star tip; a 3-inch ring cutter
For the lemon pistachio cake: Preheat the oven to 325 degrees F. Line a half-sheet tray with parchment paper.
In the bowl of a food processor, pulse the pistachios and 1/4 cup flour until the nuts are finely ground.
In the bowl of an electric mixer, cream together the butter and granulated sugar until very light. Beat in the pistachio paste and lemon zest and juice, then add the eggs one at a time. Scrape down the sides with a rubber spatula and beat in the pistachio and vanilla extracts. In a small bowl, mix together the nut and flour mixture, salt, baking powder and remaining cup flour. Beat the dry ingredients into the batter.
Spread the batter evenly in the lined half-sheet tray. Bake for 15 minutes, then rotate the tray and continue to bake until the cake springs back lightly when touched and a cake tester inserted in the center comes out clean, another 10 minutes. Allow the cake to cool in the tray for 10 minutes, then gently remove from the tray.
For the lemon curd: Combine the lemon zest, juice, granulated sugar, eggs and salt in a medium saucepan and whisk to a homogeneous consistency. Place over medium heat. Cook, whisking constantly, until the mixture has thickened, 12 to 15 minutes.
Remove from the heat and whisk in the butter, 2 pats at a time, until it's incorporated and has a silky consistency. Transfer to a bowl. Place a piece of plastic wrap over the top, pressing down on the surface of the curd to prevent a film from forming. Put in the fridge to chill for 1 hour.
For the lemon mascarpone mousse: In a large, chilled bowl, whip the heavy cream to stiff peaks. In another large bowl, whip the mascarpone and granulated sugar together until fluffy. Whip in half of the lemon curd. Fold in the whipped cream and use a rubber spatula to fluff the mousse. Transfer the mousse to a piping bag fitted with a star tip.
To assemble the cake: Cut the cake into individual cakes with a 3-inch ring cutter.
Spread a dollop of lemon curd on the bottom of a plate. Place 2 cake rounds on top. Pipe a round of mousse onto each cake.
Garnish each cake with chopped pistachios, a dusting of powdered sugar and the chopped rosemary.