Step 1

Prep the oven and baking pan. Position a rack in the center of the oven and preheat the oven to 350°F. Lightly spray an 8-inch square pan with cooking spray and line with parchment paper, leaving a 2-inch overhang on each of the pan's sides. Spray the parchment paper, too.

Step 2

Mix the dry ingredients. In a medium bowl, whisk together the flour and salt.

Step 3

Melt the butter and ruby chocolate for the batter. Place the butter and white chocolate in the top pan of a double boiler or in a heatproof bowl set over a medium, heavy-bottomed saucepan filled with a few inches of simmering water (be sure the bottom of the bowl does not touch the water).Cook over medium heat, using a heatproof rubber spatula to stir the mixture and scrape the sides of the bowl occasionally, until the chocolate and butter have melted and combined, about 5 minutes. Remove the pan or bowl from heat and set on a wire rack to Let the chocolate mixture cool slightly while you prep the other ingredients.

Step 4

Whisk the sugars, eggs, vanilla, and food coloring, then add the chocolate mixture. In the bowl of a stand mixer fitted with the whisk attachment, combine the sugars, eggs, vanilla, and food coloring. Whisk on medium-high speed for 8 minutes, or until thick, fluffy, and doubled in volume. Reduce to low and slowly pour in the warm chocolate mixture, whisking on low speed until just combined.

Step 5

Mix in the dry ingredients. Once the chocolate mixture is combined, use the rubber spatula to scrape down the whisk and the bottom and sides of the bowl. Sprinkle the dry ingredients over the batter all at once and use a rubber spatula to mix by hand until just combined.

Step 6

Assemble the blondies. Pour the batter into the prepared pan and use an offset spatula to spread it evenly across the pan and smooth the top. Sprinkle the batter with the remaining ruby chocolate for assembly.

Step 7

Bake the blondies. Bake for 30 minutes, or until a skewer inserted into the center of the blondies comes out with a few crumbs attached. Cool completely on a wire rack before slicing.

Step 8

Serve and store. Run a butter knife or the offset spatula along the edges of the pan and use the overhanging parchment as handles to lift the blondies out of the pan and onto a cutting board. Slice into 2-inch squares and serve.The blondies can be stored in an airtight container or zip-top bag at room temperature for up to 3 days.