Preheat the oven to 350 (F). In a large bowl combine softened butter and brown sugar. Stir until well combined. Add eggs and vanilla and mix until just combined.
Add flour, crushed graham crackers, and salt. Mix just until no more flour streaks appear. It should be a fairly thick batter.
Prep an 8x8" pan by either greasing it or lining it with parchment paper. Pour the batter into the prepared pan and use a spatula to even it out throughout the pan.
Bake for 25-30 minutes, until there is just a slight jiggle in the middle, or until a toothpick comes out with just a few crumbs.
Let the bars cool completely (you can put the bars in the fridge if you are in a hurry).
While the bars cool, make the topping. Add the heavy cream and the zest of one lime to a large bowl. Whip until stiff peaks form, about 3-5 minutes. If you want a stronger lime taste, you can add some juice from the lime.
Cut the bars into 9 pieces (or smaller if you'd like). Using a piping bag or cookie scoop, dollop the whipped cream onto each bar then top with a thin slice of lime and crushed graham crackers. Enjoy!