Preheat oven to 180 C / Gas 4. Grease a 2lb (900g/27.5cm) loaf tin well and dust with flour.
Beat butter, sugar and salt in a bowl till foamy. Add eggs one by one and stir well after each addition. Stir in 3/4 of the flaked almond flakes (reserve the rest) and fold in lemon juice, zest and vanilla.
Place flour and baking powder in a bowl; add the rhubarb and stir until well coated. Fold in the butter mixture. Tip the cake mixture into the prepared tin.
Mix the remaining almond flakes with 2 to 3 tablespoons sugar and distribute evenly on top. Place a few pieces of diced butter on top of the cake.
Bake in the preheated oven until a skewer comes out clean, about 65 minutes. Remove from the oven and let cool in the tin for 10 minutes; turn out onto a wire rack and let cool completely.