Chocolate Cake with Chocolate-Pecan Frosting

Preheat the oven to 350°F and prepare two 9-inch round cake pans by greasing and flouring them.

Whisk together all-purpose flour, granulated sugar, baking soda, and salt in a large bowl.

Combine unsalted butter, margarine, water, and unsweetened cocoa powder in a saucepan and heat over medium heat until the butter and margarine melt, stirring occasionally.

Pour the hot cocoa mixture into the flour mixture and stir until just combined.

Whisk together sour cream, eggs, and vanilla extract in a separate bowl until smooth.

Add the sour cream mixture to the batter and stir until fully incorporated and smooth.

Divide the batter evenly between the prepared pans and smooth the tops.

Bake for 30-35 minutes, or until a toothpick inserted in the center of each cake comes out clean.

Remove the pans from the oven and allow the cakes to cool in the pans for 10 minutes, then turn out onto wire racks to cool completely.

For the frosting, beat unsalted butter in a large bowl with an electric mixer until light and creamy.

Add unsweetened cocoa powder and milk to the butter and beat until combined and smooth.

Gradually add the sifted confectioners' sugar and vanilla extract to the cocoa mixture, beating on medium speed until the frosting reaches a spreadable consistency, then fold in the toasted pecans by hand until evenly distributed.