Preheat oven to 375°F (190°C). Grease a 15x10x1-inch baking sheet.
In a large bowl, combine flour, sugar, baking powder, baking soda, and salt. Set aside.
In a small saucepan, bring butter and water to a boil. Add to flour mixture and mix well.
Stir in sour cream, eggs, and almond extract until smooth. Pour into prepared baking sheet.
Bake for 20-22 minutes or until a toothpick inserted in the center comes out clean. Cool on a wire rack.
For frosting, in a small saucepan, bring butter and milk to a boil. Remove from heat and stir in powdered sugar and almond extract until smooth. Spread over warm cake. Sprinkle with walnuts.