2 1/4 cups / 286 g all-purpose flour, plus more for dusting the pan
5 large egg whites
1/2 tsp cream of tartar
1 1/2 cups plus 2 tbsp / 320 g granulated sugar
1 tbsp plus 1 tsp baking powder
6 large egg yolks
1/2 cup / 110 ml whole milk
1 1/2 tsp vanilla extract
For the Tres Leches
3 cups / 720 ml whole milk
2 cups / 480 ml evaporated milk
1 3/4 cups / 420 ml sweetened condensed milk
Heaping 1/2 cup / 42 g Thai tea powder (look for no added sugar or milk powder, such as Wangderm)
For the Whipped Cream
2 cups / 480 ml heavy cream
1/2 cup / 117 g sour cream
3 tbsp granulated sugar
1 tsp vanilla extract
For Serving
2 cups / 100 g unsweetened coconut flakes, toasted
3 limes, for zesting
Directions
Heat oven to 375F. Butter a 9-by-13-inch cake pan and dust with flour.
Prepare the cake: In a stand mixer with the whisk attachment, whip the egg whites with the cream of tartar on medium-high until soft peaks form, about 1 minute. Gradually add the sugar, whipping as you go, until fluffy with firm peaks, about 5 minutes.
In a small bowl, whisk together the flour and baking powder. In a medium bowl, whisk together the egg yolks, whole milk and vanilla until well combined. With the mixer running on medium, gradually add the wet ingredients to the whipped whites until combined, scraping the bowl as needed. Gradually add the dry ingredients until combined.
Scrape the batter into the prepared pan and spread evenly. Bake 18 minutes, then rotate the pan and bake another 8 to 10 minutes, until the top is golden brown and a toothpick inserted in the center comes out clean.
Let the cake cool in the pan until slightly warmer than room temperature, about 30 minutes. Refrigerate the cake, still in the pan and uncovered, while you make the tres leches.
Make the tres leches: In a small saucepan, warm the whole milk, evaporated milk and condensed milk over medium heat, stirring occasionally, until just shy of simmering (steaming but not bubbling), 10 to 15 minutes. Whisk in the tea powder, take off the heat, and let steep 10 minutes (it will turn orange). Strain through a fine-mesh sieve.
Poke holes all across the surface of the chilled cake with a fork (the more the better). With the cake still in the pan, cut it into 12 even pieces (easier now than once soaked).
Slowly and evenly pour the warm tres leches over the cake, pausing to let it absorb. The cake should be fully soaked through the center; no white crumb should be visible. Cover with plastic wrap and refrigerate overnight, or at least 3 hours.
Just before serving, make the whipped cream: Combine the heavy cream, sour cream, sugar and vanilla in a stand mixer bowl. Whip on high until doubled in volume with stiff peaks, about 1 minute.
Transfer each slice to a plate. Spread or pipe the whipped cream on top, scatter with toasted coconut until fully covered, and finely grate lime zest over each slice. Serve cold; the texture should melt in your mouth, close to ice cream.
To toast the coconut: spread on a rimmed baking sheet and bake at 350F until golden and crunchy, stirring once, 5 to 10 minutes.
From Sam Yoo at NYC's Golden Diner, adapted by Ligaya Mishan for NYT Cooking. Leftovers keep refrigerated up to 3 days.
