Start by preheating the oven to 350 F
Stir the flour, sugar, baking powder, and salt in a large bowl
Whisk together the sour cream, eggs, almond extract, and vanilla extract in a different bowl.
Melt the butter with the water in a small saucepan over medium heat, and add this to the bowl with the dry ingredients.
Pour the egg mixture into the dry ingredients and mix it all up until smooth.
Spray a 10x15-inch sheet cake with cooking spray and pour in the batter. Spread evenly and bake for 20 mins.
Prepare the frosting while the cake is baking. In the same saucepan, melt the butter with the milk and bring to a boil. Remove from heat and using a hand-held mixer, beat in the powdered sugar, vanilla, and almond extracts until smooth and all the sugar has dissolved.
Remove the cake from the oven and immediately pour the hot frosting over the warm cake. Use a spatula to smooth it over the entire surface of the cake.
Before cutting it into squares, let it cool completely and if you like, add sliced almonds on top.