Ingredients
1 stick unsalted butter, melted and cooled, plus more, softened, for pan
1 2/3 cups unbleached all-purpose flour
2 teaspoons baking powder
3/4 teaspoon kosher salt (we use Diamond Crystal)
4 large eggs, room temperature
1 1/2 cups granulated sugar
1 tablespoon finely grated orange zest
2 tablespoons poppy seeds
2 teaspoons pure vanilla paste or extract
3/4 cup sour cream, room temperature
2 cups confectioners' sugar, sifted
1 tablespoon unsalted butter, melted
1 tablespoon fresh orange juice
1 teaspoon poppy seeds
Preparation
Cake: Preheat oven to 350°F. Brush a 9-by-13-inch or 8-by-12-inch baking pan with butter. Line with parchment, leaving a 2-inch overhang on long sides. Butter parchment.
Whisk together flour, baking powder, and salt. Combine eggs and sugar in the bowl of a mixer fitted with the whisk attachment. Beat on high speed until thick and pale, about 4 minutes. Add flour mixture; beat to combine. Add butter, zest, poppy seeds, and vanilla; beat to combine. Add sour cream; beat until smooth.
Scrape batter into prepared pan, smoothing top with a spatula. Bake until top springs back when lightly touched and a tester inserted in center comes out clean, 30 to 35 minutes (for 9-by-13-inch) or about 40 minutes (for 8-by-12).
Let cool in pan on a wire rack 10 minutes. Using parchment overhangs, lift cake out of pan and let cool completely on rack. (Cake can be made to this point and stored at room temperature, wrapped in plastic, up to 1 day.)
Icing: Whisk together confectioners’ sugar, butter, and orange juice until smooth. Spread over cake. Sprinkle with poppy seeds. Let set at least 20 minutes before serving. Iced cake can be stored in an airtight container at room temperature up to 3 days.
