Prepare bars: In a large bowl with an electric mixer, cream butter and sugar until fluffy. Add eggs, one at a time, mixing after each egg. Mix in vanilla.
In a separate bowl whisk together flour, salt & baking soda. Add to wet mixture and mix just until combined. Using your hands spread dough on a greased, rimmed baking sheet (use a 13 x 18 pan). The dough will be just like sugar cookie dough- heavy & sticky.
Bake at 375 degrees for 10-15 min, until light golden brown or until a toothpick comes out clean. Cool completely before frosting.
Prepare frosting: In a large bowl, beat butter and shortening together until smooth and creamy. Mix in vanilla (and almond extract if using). Add powdered sugar (1 cup at a time) until combined, then add milk (I like to use cream) & mix until smooth and spreading consistency. Add a few drops of color, if desired. Spread over cooled cookie, then cut into bars.