Preheat your oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper, leaving an overhang for easy lifting. Lightly grease the pan if your parchment won't fully cover the sides.

In a large bowl, whisk together the flour, powdered sugar, and salt. Add cold cubed butter and, using a pastry cutter, two forks, or your clean fingertips, cut the butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized pieces. Stir in the vanilla.

Press about two-thirds of the crust mixture firmly and evenly into the bottom of the prepared pan to form a compact base. Reserve the remaining third for a crumble topping. Bake for 15–18 minutes, or until the edges are lightly golden. Remove from oven and set aside.

While the crust bakes, place raspberries, sugar, and lemon juice into a small saucepan over medium heat. Cook, stirring, until raspberries break down and the mixture becomes saucy (5–8 minutes). If you want a thicker swirl that won’t bleed into the chocolate too much, dissolve 1 teaspoon cornstarch in a splash of water and stir in, cooking for another minute. Remove from heat and let cool slightly.

For a seedless finish, press through a fine mesh sieve—this step is optional but lovely if kids or guests don't love seeds.

In a heatproof bowl, combine chopped dark chocolate and butter. Heat the cream in a small saucepan until it just starts to simmer (do not let it boil). Pour the hot cream over the chocolate and let sit 30–60 seconds, then gently whisk until glossy and smooth. Add sugar only if you prefer a sweeter bar, then swirl in vanilla.

Pour the chocolate filling over the baked crust and spread gently to an even layer. Dollop spoonfuls of the raspberry sauce across the chocolate, then use a knife or skewer to swirl the raspberry into the chocolate in a loose, pretty pattern—don’t overmix or the swirl will disappear.

Sprinkle the reserved crust crumbs evenly over the top. The crumbs will bake into a slightly crunchy top during the finishing bake.

Bake at 350°F (175°C) for another 12–15 minutes, until the crumb topping is lightly golden and the chocolate has set around the edges. Cool completely on a wire rack, then chill in the refrigerator for at least 2 hours to fully set the chocolate (overnight is even better).

Lift the bars from the pan using the parchment overhang and slice into bars. Sprinkle with flaky sea salt or powdered sugar and garnish with fresh raspberries if desired.