Photograph: Laura Edwards
Preheat the oven 180°C, fan 160°C, gas 4. Grease a 22cm springform tin and line the base and sides with baking paper.
Put the butter and sugar in a bowl and cream together with an electric whisk until pale and fluffy. Whisk in the eggs one at a time until creamy. Add the ground almonds and orange zest, sift over the flour and baking powder and mix well.
Spoon half the mixture into the tin, carefully spreading it right to the edges. Arrange just under half the rhubarb on top, keeping it away from the edges of the tin. Sprinkle over 1 tablespoon of the extra sugar.
Spoon over the rest of the cake mixture, then spread it to cover the rhubarb. Arrange the rest of the rhubarb on top, in a circle, filling in any gaps (keeping it away from the edges). Scatter over the flaked almonds and sprinkle with the remaining extra sugar.
Bake for 1 hour until golden, cover with a tent of foil and bake for another 10-15 minutes until firm to the touch in the middle. Leave in the tin to cool before transferring to a wire rack. If not serving warm, allow to cool completely.
Get ahead: the cake can be stored in an airtight container for up to 3 days in a cool place.