Cosmic Blondies Are Out of This World

INGREDIENTS

For the blondies

• 1 cup/227 grams unsalted butter, plus more for greasing
• 2 ice cubes
• 1 tablespoon vanilla extract
• 2 ⅔ cups/340 grams all-purpose flour
• 1 teaspoon baking soda
• 1 teaspoon kosher salt (such as Diamond Crystal), or ½ teaspoon fine salt
• 1 cup/220 grams dark brown sugar
• ½ cup/100 grams granulated sugar
• 2 large eggs
• ¾ cup/4 ounces/113 grams chopped semisweet chocolate chunks or chocolate chips
• ¾ cup/4 ounces/113 grams mini M&M’s

For the ganache

• ½ cup/120 milliliters heavy cream
• 12 ounces/340 grams chopped white chocolate (about 2 cups; see Tips)
• Pinch of salt
• 2 food colorings of choice, preferably gel paste food coloring
• Sanding sugar (optional), for decorating
• Flaky sea salt, for sprinkling

DIRECTIONS

1. Make the blondies: Heat the oven to 350 degrees. Butter a 9-by-13-inch baking pan, (preferably metal because it conducts heat better), and line the bottom with parchment paper, leaving an overhang on the long sides. (These become your “handles” to make the blondies easier to remove once cool.) 
2. In a medium saucepan, melt the butter over medium. Once it begins to bubble vigorously, cook for another 4 to 6 minutes, swirling occasionally, until the bubbles subside and turn into foam, and toasty brown flecks begin to float on the surface. Take off the heat and scrape the bottom of the pan. Carefully add the ice cubes and stir to melt (the mixture will bubble rapidly at first). Pour the browned butter into a heatproof liquid measuring cup; it should measure 1 cup (and if not, you can supplement with water). Stir in the vanilla extract, and set the brown butter mixture aside.
3. In a medium bowl, whisk together the flour, baking soda and salt. In a separate large bowl, whisk both sugars and the brown butter mixture for about 1 minute until the sugar starts to dissolve and the mixture looks less grainy. Add the eggs one at a time, whisking heartily after each addition to make sure they’re well combined. 
4. Add the flour mixture to the sugar mixture and using a flexible spatula, stir until all but a few pockets of the flour are absorbed. Fold in the chocolate until evenly distributed and no floury bits remain. Scrape the batter into the prepared pan and spread into an even layer.
5. Bake for 22 to 24 minutes, until the top looks set and the sides begin to pull away from the pan’s edges. Remove the pan from the oven and gently tap it a couple times on a clean work surface to create a chewier texture, then let the blondies cool completely in the pan. Transfer to a cutting board for decorating and cutting.
6. Make the ganache: In a medium saucepan, heat the heavy cream over medium until small bubbles start to form around the edges and steam starts rising from the surface, 3 to 5 minutes. Remove from the heat and add the white chocolate and salt. Let the chocolate melt in the hot cream for a minute, then whisk until no chunks remain.
7. Dividing evenly, quickly pour two-thirds of the ganache into 2 separate small bowls, leaving the remaining one-third in the saucepan. Tint the chocolate in the bowls with food coloring (see Tips). Pour alternating dollops of the colored ganache all over the surface of the cooled blondies, then dollop the plain white ganache to cover the empty spaces. Using an offset spatula or the back of a spoon, swirl and smooth the colors until the desired effect is achieved and the blondies are fully covered. Lightly sprinkle sanding sugar over the surface, if using, then lightly dust the surface with flaky sea salt. Allow the ganache to set, either 30 minutes at room temperature or 15 minutes in the fridge.
8. Use the parchment sling to lift the blondies from the pan to a cutting board. Cut the blondies into squares and serve immediately, storing any leftovers in an airtight container in a single layer at room temperature for up to 3 days.

NOTES

• Try adding any of your favorite mix-ins, such as toasted nuts or sprinkles.
• Make sure to use real white chocolate and not white baking chips (cocoa butter should be an ingredient) for the ganache, as the baking chips could cause your ganache to seize. 
• For the most dramatic swirls, avoid choosing contrasting colors for your two bowls of colored ganache. Blue and orange, for example, would create brown when swirled together. Instead, choose two colors that wouldn’t make brown if combined, like pink and purple, yellow and green, or red and blue. Gel food coloring intensifies as it sits, so start with just a couple drops; you can always add more!