Preheat oven to 375° and grease or parchment line an 18x13* (see notes on 9x13) inch pan (½ sheet pan).
Cream butter for 2-3 minutes. Add sugar and beat until until light and fluffy, occasionally scraping down sides. If using natural cane sugar, mix on high speed for at least 2-4 minutes. Add eggs, one at a time, mixing well (about 1 minute) after each addition, scrape bowl after each addition.
While eggs are mixing, in medium bowl, measure out flour, baking soda and salt and whisk together. Once eggs are creamed in, scrape down sides of bowl again, add vanilla and mix well. Slowly pour in flour, mixing on low speed until all the flour is incorporated, scrape down sides of bowl. Mix on medium speed for 30 seconds.
Spread dough into prepared pan, using a non-stick sprayed piece of plastic wrap or parchment paper, spread over top of of dough and spread evenly to all corners of the pan, making sure it's as even as possible.
Bake at 375° for 10 minutes, no longer, they will not look done, remove from oven and let cool completely. May be wrapped and frozen (cool completely). If making in 9x13 pan you may need any extra 1-2 minutes but no longer, you want it puffy, but should look dull, no shiny parts.
MILE HIGH BUTTERCREAM FROSTING
While cookies cool (or take out of freezer, if frozen) make frosting.
Place butter in the mixer bowl and whip on medium-high speed until light & fluffy, 2-4 minutes. Add the vanilla and pinch of salt and whip up well. Gradually add powdered sugar, mixing on low. Pour 4 tablespoons (with mixer running) of milk or cream and whip up well, depending on consistency needed (less milk for piping) a little extra for spreading typically.
Add more milk as needed, whip for 4-8 minutes until cloud like light! If you'd like to color the frosting, add tiny amounts of gel food coloring until desired tint is achieved. If adding an extract, add at end and whip for additional minute. Spread or pipe onto baked goods. Add sprinkles, if desired
Notes
*If thicker bars and frosting are desired, bake in parchment lined 9x13 pan, for about 11-12 minutes, if you want underdone (which I love) cookies. Cool completely and frost.TIP | To make it easier to cut, freeze the bars and frosting for 20 minutes, using the parchment lift out of of pan and onto cutting board, using a long knife, running under hot water and wiping before each cut will ensure a smooth and clean cut.