Vegan Chocolate Cake With Chocolate-Orange Frosting

Place rack in middle of oven and preheat to 350°F. Spray 9" round cake pan with 2"-high sides with nonstick vegetable oil spray. Line pan with parchment paper and spray parchment.

Sift 1 cup all-purpose flour, 1 cup sugar, ½ cup whole wheat pastry flour, 6 Tbsp. unsweetened cocoa powder, 1 tsp. baking soda, and ½ tsp. Diamond Crystal or ¼ tsp. Morton kosher salt into a large bowl.

Add ¾ cup water, ⅓ cup vegetable oil, ¼ cup frozen orange juice concentrate, thawed, 1 tsp. vanilla extract, and 1 tsp. distilled white vinegar or apple cider vinegar to bowl with dry ingredients and whisk to blend. Transfer batter to prepared pan.

Bake cake until toothpick or other tester inserted into center comes out clean, about 35 minutes. Transfer cake pan to wire rack to cool completely, about 1½ hours. Carefully cut around sides of pan and turn cake upside down onto a serving platter.

Place 8 oz. bittersweet chocolate (do not exceed 61% cacao), chopped in medium heatproof mixing bowl. Set bowl over saucepan of barely simmering water. Stir constantly until chocolate is melted and smooth. Carefully transfer bowl to work surface. Cool melted chocolate to room temperature, stirring occasionally, 15 to 20 minutes.

Beat 6 Tbsp. (3 oz.) vegan butter (such as Earth Balance) or margarine, room temperature in the bowl of a stand mixer fitted with the paddle attachment (or with a handheld electric mixer) on medium speed until smooth. Add 1 cup powdered sugar, 1 Tbsp. frozen orange juice concentrate, thawed, ½ tsp. vanilla extract, and 1 large pinch kosher salt and continue beating until smooth. Add remaining 2 Tbsp. frozen orange juice concentrate, thawed, and ¼ cup light agave nectar and beat until blended. Add chocolate in a slow, steady stream, beating until fully incorporated. Let stand until slightly thickened, about 10 minutes, before frosting top and sides of cake. (Frosting will firm up, so don’t let it rest too long before frosting cake.) Do ahead: Cake can be made up to 8 hours ahead. Cover with cake dome and store at room temperature.