Remove from heat and whisk in flour, baking soda, salt, sugar, eggs, sour cream and almond extract until smooth.
Pour into a greased 11x15 baking pan. Bake at 375 degrees for 20-22 minutes or until a toothpick inserted in center comes out clean. Cool in pan on a wire rack for 20 minutes.
While cake is cooling, make frosting. Combine butter and milk in a saucepan; cook over low heat until butter melts. Bring to a boil over medium heat.
Remove from heat and add powdered sugar and almond extract; beat at medium speed with an electric mixer until spreading consistency. Stir in walnuts (optional).
Spread frosting on top of cake.