Croatian Honey Cake

Steps

1. Make the honey dough: Sift the 14.1 ounces plain flour with the 1 tablespoon bicarbonate of soda (baking soda) three times total for an extra-light dough. In a saucepan, warm the 1.5 fluid ounces whole milk (for dough), 1.5 fluid ounces vegetable oil, 1.1 ounces light runny honey (for dough), and 5.3 ounces caster sugar (for dough) over low heat, stirring, until the honey and sugar dissolve. Cool until just warm to the touch, then mix in the flour and 1 medium egg, lightly beaten by hand until it comes together into a soft dough. Wrap in plastic and chill for 15 minutes.
2. Roll out the dough: Line two 37cm x 28cm (about 14.5in x 11in) Swiss roll tins with parchment. Split the chilled dough in half and roll each piece out on parchment, then lift it (paper and all) into a tin.
3. Preheat the oven to 180°C/350°F (gas mark 4): Get the oven ready.
4. Bake the sponge layers: Bake both tins for about 5 minutes, just until barely set — firm to the touch but still pale. Don't overbake or the layers turn brittle. As soon as they come out, cut each sheet in half across its width so you end up with 4 rectangular layers. Let them cool in the tins.
5. Start the honey custard: Split the 1 vanilla pod lengthwise and add it to a saucepan with the 17 fluid ounces whole milk (for custard), 2.7 ounces runny honey (for custard), and 2.7 ounces caster sugar (for custard). Bring to a boil, then remove from heat. Whisk in the 6 tablespoons fine semolina, return to low heat, and whisk constantly until thickened. Off the heat, scrape the vanilla seeds into the mixture and discard the pod.
6. Cool and enrich with butter: Transfer the custard to a bowl, press plastic wrap directly on the surface, and let it cool until just warm. Whisk in the 7.1 ounces unsalted butter, diced a little at a time, fully incorporating each addition before adding more.
7. Make the chocolate custard: Cover the custard again and let it finish cooling. Melt the 3.5 ounces dark chocolate, chopped (microwave or over a pan of barely simmering water). Scoop out about a third of the custard into a separate bowl and stir the melted chocolate into it — this becomes your chocolate layer. Keep both custards covered until ready to use.
8. Make the rum syrup: Bring the 3.5 ounces caster sugar (for rum syrup) and 3.5 fluid ounces water to a boil in a small pan, stirring to dissolve the sugar. Remove from heat and stir in the 1.7 fluid ounces dark rum.
9. Start assembling: Set the first sponge layer on a cake board or platter and brush it generously with about a quarter of the warm rum syrup. Spread half the honey custard evenly on top.
10. Build the middle layers: Add the second sponge layer, press down gently, brush with more syrup, then spread the chocolate custard over it. Add the third layer, press down, and brush with more syrup.
11. Finish layering: Spread the rest of the honey custard on top, place the final sponge layer on, and brush with the remaining syrup. If your layers are baked a bit too firm, cover the assembled cake and refrigerate overnight — it softens up nicely.
12. Trim and serve: Trim the edges neatly with a sharp knife, slice into 8 rectangles, and dust with a little 1 tablespoon icing sugar, for dusting before serving.