• Preheat oven to 325°F with a rack positioned in the center. Grease a 9x13-inch glass baking dish.

Make The Cake:

• In a large mixing bowl or the bowl of a stand mixer with the paddle attachment, beat butter and sugar on medium speed until very light and fluffy, 3-5 minutes, scraping down the bowl once or twice.
  
• Add the eggs one at a time, beating until smooth after each addition.
  
• Add the lemonade concentrate and vanilla, and beat until well combined. The mixture may look slightly curdled, which is fine.
  
• In a separate bowl, whisk together the flour, baking powder, baking soda, and salt until no lumps remain.
  
• With the mixer running on low speed, gradually add 1/3 of the flour mixture and mix just until no dry streaks remain.
  
• Add 1/2 of the buttermilk and mix just until well combined.
  
• Repeat with another 1/3 of the flour mixture, the remaining buttermilk, and the final 1/3 flour, mixing on low and stopping to scrape down the bowl as needed. Do not overmix.
  
• Transfer the batter to the prepared baking dish and use an offset spatula to smooth the top. Bake for 35-40 minutes, until the top is golden, the center springs back lightly when touched, the edges begin to pull away from the pan, and a toothpick inserted in the center comes out with a few moist crumbs but no wet batter.
  
• Cool completely in the baking dish on a wire rack, about 1 hour, before frosting.

Make The Frosting:

• In a large mixing bowl or the bowl of a stand mixer with the paddle attachment, beat the butter on medium speed until smooth, creamy, and slightly lightened in color, 2-3 minutes, scraping down the bowl halfway through.
• Add 1 cup of powdered sugar, 1/3 cup of the lemonade concentrate, and the vanilla extract. Mix on low speed until the sugar is mostly moistened, then increase to medium and beat until smooth.
  
• With the mixer on low speed, gradually add 2 more cups of powdered sugar, mixing until fully incorporated and the frosting is thick and smooth. Add up to 1 more cup powdered sugar, a little at a time, if you prefer a stiffer frosting. Beat on medium speed until light and fluffy, 1-2 minutes.
  
• If the frosting is too thick, add the remaining 2 tablespoons of lemonade concentrate, 1 tablespoon at a time. If it becomes too thin, add more powdered sugar, a few tablespoons at a time, until it is smooth and spreadable.

Finish And Serve The Cake:

• Once the cake has completely cooled to room temperature, spread the frosting overtop in a thick, even layer, swirling the top as desired.
  
• Slice and serve immediately or cover and refrigerate.