To make the crust, preheat oven to 350 degrees F and line a 9x13 inch pan with parchment paper.

In the bowl of an electric mixer with a paddle attachment, combine the 18 tbsp. softened butter, 1/4 cup sugar, and 1/4 cup brown sugar. Beat on medium-high speed until smooth, 1-2 minutes. With the mixer on low speed, mix in the 2 cups of flour and 1/2 tsp. salt until just incorporated.

Add the dough to the prepared baking pan. Press into an even layer over the bottom of the pan (I found it helpful to grease my hands with a bit of Pam while doing this). Bake for about 25 minutes, or until light golden brown. Remove from the oven, maintaining the oven temperature.

While the crust bakes, make the top layer. Whisk together the sugar, flour, lemon zest, and salt in a large bowl. Add the raspberries to a fine mesh sieve and press through over a bowl, mashing with a spatula to extract as much juice and pulp as possible, straining out the seeds.

Add the egg whites and eggs to the bowl with the dry ingredients and whisk well to blend. Whisk in the raspberry puree and lemon juice until smooth (Batter will be almost completely liquid, fear not!). Pour the mixture over the crust.

Bake until the center is just set and only jiggles the tiniest bit when gently shaken, about 35-40 minutes (The top will likely become duller in color and your sides might brown, that's ok!). Transfer to a wire rack to cool to room temperature.

Cover and chill well in the refrigerator, at least 2 hours.

When ready to serve, use the parchment paper to lift the bars from the pan (You might have to run a knife around the edge a couple times if your filling leaked through the parchment a bit and is sticking to the pan.).

Place on a cutting board and slice into bars. If your browned edges got a little over done, I recommend trimming them off so they don't add a weird chewy texture. Dust the tops with powdered sugar if desired.

Store in the refrigerator.