1. Preheat oven to 325.
2. In a large bowl, add 8 tablespoons (1 stick) melted butter, 1/4 cup sugar, 1 teaspoon vanilla and 1/4 teaspoon salt. Mix well until combined. Add 2 cups flour and stir until everything comes together and you have a soft dough. Press dough into a buttered or parchment-lined 9x9 pan in a single layer.
3. Bake for 16 minutes or until the edges start to turn light brown. As soon as you take the pan out of the oven, use a fork to poke holes all over the crust (but don't go all the way through to the bottom of the pan!). 
4. While the crust bakes, make the filling by whisking together 4 eggs, 6 tablespoons lemon juice, 6 tablespoons rosé and lemon zest (if using). Sift in 1 1/2 cups sugar and 1/4 cup flour and mix to combine.
5. Pour filling over warm crust. Place pan back in oven and bake for 20 minutes or until center is set. The pan might still be a little jiggly, but use the center as a guide. 
6. Let bars cool completely before serving. Dust a little powdered sugar on top, if desired. 
7. Store leftover lemon rosé bars in the fridge for 5-7 days. They can also be frozen easily - place cut bars on a baking sheet and freeze for at least 1 hour. Individually wrap each frozen bar in plastic wrap or parchment paper and then store them all in a large bag or container.

Recipe adapted from Sally's Baking Addiction