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1
Heat the oven to 175 °C. Butter and line two 22 cm round cake tins. Sift the buckwheat flour, plain flour, baking powder and salt together into a bowl and set aside, so the buckwheat flour — which can carry small gritty lumps — is fully aerated and evenly combined with the leavening before it meets the batter.
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2
Beat the softened butter and caster sugar together with a stand or hand mixer until the mixture is very pale, almost white, and noticeably increased in volume — a full 5 to 7 minutes. This is where the dish lives or dies: buckwheat batter receives little lift from leavening alone, and the air beaten in here is the cake's primary structure. The mixture must feel light and hold a soft ribbon.
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3
Add the eggs one at a time, beating well after each addition and scattering a tablespoon of the sifted flour over the batter before each new egg, so the flour steadies the emulsion and the fat-heavy batter does not split into a greasy, curdled mass. If using rum or pear schnapps, add it with the final egg.
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4
Fold in the remaining flour mixture in two additions using a large spatula, working gently and stopping as soon as no dry flour is visible, so the air you have built into the batter is not knocked out by overworking. The batter will be dense and dark; divide it equally between the two prepared tins and smooth the surfaces.
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5
Bake at 175 °C for approximately 35 minutes. Buckwheat batter darkens and appears set before it is cooked through, so do not trust the colour — the reliable test is a thin skewer inserted into the centre, which must come out clean and dry with no moist crumb clinging to it.
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6
Remove from the oven and leave the layers to cool in their tins for 10 minutes, then turn out onto a wire rack, so the structure firms enough to hold its shape without crumbling at the edges. Allow to cool completely before sandwiching — if the layers are still warm, the jam will run straight off the crumb and the filling will be lost.
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7
Place one layer on a serving plate and spread the bilberry jam evenly across the entire surface to the edges. Set the second layer on top, press gently, and dust the surface generously with icing sugar. The cake is ready to serve immediately but improves markedly after 24 hours, as the jam moistens and softens the dense crumb.