Prepare the Pan:
Preheat oven to 350°F (175°C). Line a 9x9-inch baking pan with parchment paper, leaving overhang on two sides for easy removal. Lightly grease the parchment.

Make the Dark Chocolate Batter:
Combine bittersweet chocolate and butter in a heatproof bowl. Microwave in 30-second intervals, stirring between each, until melted and smooth. Alternatively, melt over a double boiler. Let cool 5 minutes. Whisk in sugar until combined; the mixture will be grainy. Add eggs one at a time, whisking well after each addition until the batter becomes glossy and slightly thickened. Whisk in vanilla, espresso powder, and salt. The espresso deepens the chocolate flavor without adding coffee taste. Fold in flour with a spatula until just combined. Do not overmix. Set aside.

Make the Ruby Chocolate Batter:
Ruby chocolate is temperamental and can seize if overheated. Melt ruby chocolate and butter together very gently: microwave in 20-second intervals at 50% power, stirring between each, until just melted. Or use a double boiler over barely simmering water. Whisk in sugar. Because ruby chocolate is already quite sweet, this batter uses less sugar than the dark. Add egg and whisk until smooth and glossy. Whisk in vanilla and salt. Fold in flour and freeze-dried raspberry powder if using. The raspberry powder enhances both color and adds subtle tartness that balances ruby chocolate's sweetness. Set aside.

Create the Marble:
Pour about two-thirds of the dark chocolate batter into the prepared pan and spread evenly. Dollop spoonfuls of ruby chocolate batter randomly over the dark batter, about 8-10 dollops. Dollop remaining dark chocolate batter in the gaps. Using a butter knife, skewer, or chopstick, drag through both batters in a swirling pattern to create marbling. Work in both directions: horizontal swirls, then vertical, then a few figure-eights. Do not overswirl or the colors will muddy together. Stop while you can still see distinct swirls.

Bake:
Bake 28-32 minutes until the top is set and a toothpick inserted in the center comes out with moist crumbs, not wet batter. The edges should be slightly pulling away from the pan. Do not overbake. Ruby chocolate areas may look slightly underdone; this is correct. They will set as the brownies cool.

Cool and Finish:
Cool completely in pan on a wire rack for at least 2 hours, preferably 3. Patience is essential for clean slicing. Using parchment overhang, lift brownies from pan onto a cutting board. Sprinkle with flaky sea salt and crushed freeze-dried raspberries if using. Using a sharp knife dipped in hot water and wiped clean between cuts, slice into 16 squares.