Thai Tea Tres Leches Cake

Preparation

1. Step 1

   Heat oven to 375 degrees. Butter a 9-by-13-inch cake pan and dust with flour.

2. Step 2

   Prepare the cake: In the bowl of a stand mixer fitted 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.

3. Step 3

   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 speed, gradually add the wet ingredients to the whipped whites until well combined, stopping and scraping the bowl as necessary. Gradually add the dry ingredients until well combined, scraping the bowl as necessary.

4. Step 4

   Scrape the batter into the prepared pan, then spread evenly. Bake for 18 minutes, then rotate the pan and continue baking for another 8 to 10 minutes, until the top is golden brown. A toothpick inserted into the center should come out clean.

5. Step 5

   Remove the cake from the oven and let 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 mixture.

6. Step 6

   Make the tres leches: In a small saucepan, warm the whole milk, evaporated milk and condensed milk over medium heat. Stir occasionally until just shy of simmering, steaming but not bubbling, 10 to 15 minutes. Whisk in the tea powder. Take the saucepan off the heat and let the tea steep for 10 minutes. (It will turn orange.) Using a fine-mesh sieve or tea strainer, strain into a liquid measuring cup or medium bowl.

7. Step 7

   Take the cake out of the refrigerator. Using a fork, poke holes all across the surface of the cake — the more holes the better — as close together as possible. With the cake still in the pan, cut it into 12 even pieces. (It will be more difficult to cut later, once the tres leches mixture has soaked through.)

8. Step 8

   Carefully pour the warm tres leches mixture slowly and evenly over the cake, pausing as necessary to let the cake absorb the mixture. The cake should be fully soaked through the center; the white of the crumb should not be visible. Cover the cake with plastic wrap and refrigerate overnight or for a minimum of 3 hours.

9. Step 9

   Just before serving, make the whipped cream: Combine the heavy cream, sour cream, sugar and vanilla extract in the bowl of a stand mixer. Whip on high speed until the cream doubles in volume and forms stiff peaks, about 1 minute.

10. Step 10

    When ready to serve, take the cake out of the refrigerator. Transfer each slice to a separate plate. (Leftovers can be refrigerated for up to 3 days.) Using a spatula, spread the whipped cream on top of each slice, or pipe it from a pastry bag. Scatter with the toasted coconut flakes until the whipped cream is covered completely. Finely grate lime zest directly over each slice. Serve cold: The cake should melt in your mouth, with a texture close to ice cream.