• Preheat the oven to 350°F. Grease a 10x15-inch metal baking pan with butter.

Make the batter:

• In a stand mixer fitted with the whisk attachment, beat the oil, eggs, sugar, salt, and vanilla on low speed for 3 minutes. Add the oil and mix until combined.
• With the mixer on low, add the flour in three parts, alternating with the milk, starting and ending with flour. Mix just until the batter is smooth and no dry streaks remain.
• Pour the batter into the prepared pan and spread it evenly. Bake for 25 to 30 minutes, until a toothpick inserted in the center comes out clean.

Make the raspberry spread while the cake bakes:

• In a saucepan over medium heat, combine the frozen raspberries, sugar, and salt. Simmer, stirring often and breaking up the berries, until thick and jammy, about 10 to 15 minutes. It's ready when a spoon dragged across the bottom of the pan leaves a trail. Remove from the heat and stir in the lemon zest.
• As soon as the cake comes out of the oven, spread the raspberry mixture evenly over the hot cake. Let it cool completely in the refrigerator.

Make the chocolate topping:

• Melt the chocolate in a double boiler, in a heatproof bowl set over simmering water, or in the microwave in 30-second bursts, stirring between each. Stir in the oil until smooth.
• Pour the chocolate over the raspberry layer and spread it evenly. Let it sit for 2 to 5 minutes, until slightly tacky. Then sprinkle it with lightly crushed freeze-dried raspberries (if using) and flaky salt, and press them gently so they stick.
• Refrigerate until the chocolate is firm.
• Cut into bars with a sharp knife, warmed under hot water and wiped dry between cuts. For softer chocolate, let the bars sit at room temperature for about 30 minutes before serving. Store in the refrigerator for up to 3 days. The freeze-dried raspberries will soften over time, so for the best crunch, add them the day you serve.