1. Cut parchment paper to fit the baking pan (18cm x 18cm x H6cm here). Scatter the sugar crystals over the base of the mold. Set aside.
2. In a small cup, combine honey, glucose syrup and boiling water until smooth. Set aside.
3. Lightly beat the eggs and sugar (white and brown) in a heat-safe bowl, then set over a water bath, beat with an electric hand mixer (at medium speed, not high) until the egg mixture reaches 30°C on a thermometer.
4. Remove from the hot water bath, and continue beating for about 2 minutes at lowest speed.
5. Pour honey mixture from #2 and continue mixing until well combined at lowest speed.
6. Pre-heat the oven to 170℃.
7. Add sifted flour in 3 times. Beat well (at lowest speed) at each addition. Pour the cake batter over the crystal sugar in the prepared mold. Run a spatula through the mixture to remove any air bubbles, and smoothen the surface of the batter with a pastry scraper.
8. Bake the castella for 2 minutes at 170℃, remove from the heat and run a spatula through the mixture to remove any air bubbles. Put it back to the oven and repeat this process 2 more time.
9. Now bake the castella for 10 minutes at 170℃. Then reduce the heat to 150℃, and bake for about 30 to 40 minutes (in doubt insert a thin skewer in the cake, it should come out clean).
10. Remove from the oven and drop down the castella cake (with the mold) over your worktop from about 15cm high. Remove from the cake mold.
11. Place saran wrap over the top of castella tightly and flip the castella cake. Cover the cake with saran wrap completely (all sides) and let it sit wrapped overnight, keeping the top side facing down. This is at room temperature.
12. Unwrap the castella, gently peel off parchment paper. Slice off the sides of the castella with a sharp bread knife and cut castella into 2 halves. Then cut in (approx) 3cm thick slices.
About the edges of the castella, leftover from the slicing at the end : don't throw them away, you can make delicious rusks with :) 1. Place the edge of castella onto a baking sheet, and bake them for 10 minutes at 160℃ (pre-heated). 2. Turn off the oven (but do not open the door yet), leave for 8 to 10 minutes. Finally remove from the oven and spread on a rack until they cool down. It will be so crispy!