Instructions

for the cake

Adjust oven rack to middle position and heat oven to 350 degrees. Grease three 9-inch round cake pans, line with parchment paper, grease parchment, and flour pans. Whisk flour, baking powder, and salt together in bowl.
Using stand mixer fitted with whisk attachment, whip eggs and yolk on medium speed until foamy, about 1 minute. Add 2 cups sugar and orange zest, increase speed to high, and whip until mixture is fluffy and pale yellow, about 3 minutes.
Reduce speed to medium and, with mixer running, slowly pour in oil. Mix until oil is fully incorporated, about 1 minute. Reduce speed to low, add half of flour mixture, and mix until incorporated, about 1 minute, scraping down bowl as needed. Add milk and ¼ cup Grand Marnier and mix until combined, about 30 seconds. Add remaining flour mixture and mix until just incorporated, about 1 minute.
Divide batter evenly among prepared pans and smooth tops with rubber spatula. Bake until cake is deep golden brown and toothpick inserted in center comes out with few crumbs attached, 25 to 30 minutes. Transfer pans to wire rack and let cool for 15 minutes. Loosen cakes from pans with paring knife, then invert onto greased wire rack and discard parchment. Invert cakes again and let cool completely on rack, about 1½ hours.
Bring ¼ cup water, remaining 2 tablespoons sugar, and remaining ¼ cup Grand Marnier to boil in small saucepan over medium heat; cook until sugar dissolves, about 3 minutes. Let cool completely, about 30 minutes. Once cooled, brush top and sides of cake layers with syrup.

for the cranberry curd

Meanwhile, cook cranberries, sugar, orange zest and juice, and salt in medium saucepan over medium-low heat, mashing occasionally with potato masher, until cranberries have mostly broken down and mixture measures 1¼ cups, 10 to 12 minutes. Strain cranberry mixture through fine-mesh strainer into bowl, pressing on solids with rubber spatula to extract as much puree as possible. Discard solids.
Whisk eggs and yolks into cranberry mixture in bowl. Return mixture to saucepan and cook over medium-low heat, stirring constantly with rubber spatula, until mixture is thickened and registers 175 degrees in multiple spots, 5 to 7 minutes.
Off heat, stir in butter until incorporated. Transfer curd to bowl, press piece of plastic wrap directly onto surface of curd, and refrigerate for at least 3 hours. (Curd can be refrigerated for up to 3 days.)

for the garnish

Place ¾ cup sugar in shallow dish; set aside. Bring 1 cup water and remaining 1 cup granulated sugar to boil in medium saucepan over high heat; cook, stirring constantly, until sugar dissolves, about 3 minutes. Off heat, dip rosemary sprigs in sugar syrup, then roll in sugar in dish. Transfer to plate.
Stir cranberries into syrup and let syrup cool completely, about 30 minutes. (Cranberries in syrup can be refrigerated for up to 24 hours.) Drain cranberries in fine-mesh strainer; discard syrup. Working in 3 batches, roll cranberries in sugar in dish and transfer to large plate. Let stand at room temperature to dry, about 1 hour.

for the buttercream

Using stand mixer fitted with whisk attachment, whip butter on medium-high speed until smooth, about 20 seconds. Add sugar and salt and mix on medium-low speed until most of sugar is moistened, about 45 seconds. Scrape down bowl. Add cream and vanilla and whip on medium-high speed until light and fluffy, about 4 minutes, scraping down bowl as needed.
Fill pastry bag fitted with a ½-inch-wide straight tip with 1 cup buttercream. Place 1 cake layer on cake turntable. Using half of buttercream, pipe ½-inch-high ring of buttercream around top perimeter of cake layer. Spread half of cranberry curd (about 1 scant cup) evenly inside buttercream ring.
Place second cake layer on top, pressing lightly to adhere. Pipe another ½-inch-high ring of buttercream around top perimeter of cake layer with remaining buttercream in pastry bag. Spread remaining cranberry curd evenly inside buttercream ring. Top with remaining cake layer, pressing lightly to adhere. Refrigerate to allow buttercream to firm up, about 30 minutes.
Spread ¾ cup buttercream evenly over top of cake. Spread remaining buttercream evenly over sides of cake to cover with thin coat of buttercream. Run edge of offset spatula around cake sides to create sheer veil of buttercream so cake beneath is visible. Sprinkle ½ cup cranberries around bottom of cake. Pile remaining cranberries on top of 1 side of cake in half-moon shape. Place rosemary sprigs at base of cranberries. Serve.