Preheat oven to 325°F (170°C).

For cake: In the bowl of a stand mixer, whisk together granulated sugar and orange zest by hand until combined and fragrant. Add butter; using the paddle attachment, beat at medium speed until light and fluffy, 4 to 5 minutes, stopping to scrape sides of bowl. Add eggs, one at a time, beating until combined after each addition and stopping to scrape sides of bowl. Beat in vanilla.

In a medium bowl, whisk together flour, salt, and baking powder. In a small bowl, whisk together ricotta and orange juice. With mixer on low speed, add flour mixture to butter mixture in three additions alternately with ricotta mixture, beginning and ending with flour mixture, beating until just combined after each addition and stopping to scrape sides of bowl. Fold in pistachios and chocolate chips.

Spray a 10-cup fluted tube pan with baking spray with flour. Spread batter into prepared pan. Firmly tap pan on a kitchen towel-lined counter to settle batter and release any air bubbles.

Bake until golden brown and a wooden pick inserted near center comes out clean, 1 hour to 1 hour and 10 minutes. Let cool in pan for 10 minutes. Invert cake onto a wire rack, and let cool completely.

For topping: In the bowl of a stand mixer fitted with the whisk attachment, beat all ingredients at medium-low speed until thick and pipable, 2 to 3 minutes. Spoon topping into a pastry bag fitted with a medium open star piping tip (Ateco #824), and pipe onto cake as desired. Garnish with pistachios and chocolate chips, if desired. Refrigerate in an airtight container for up to 3 days.