Preheat the oven to gas 4, 180°C, fan 160°C. Grease and line a 26 x 17cm baking tin with nonstick baking paper. Put the butter and sugars in a large saucepan and warm over a medium-low heat, stirring occasionally, until the butter has melted and the sugar has dissolved. Set aside to cool for 5 mins.
Whisk in the eggs and the vanilla and almond extracts until well combined. In a separate bowl, mix the ground almonds, flour, baking powder and a pinch of fine salt together, then fold into the batter followed by the white chocolate and half the flaked almonds. Pour into the prepared baking tin.
Dollop the jam all over, then swirl into the mixture. Scatter 25g almonds on top, then bake for 35-40 mins until golden but still with a slight wobble in the centre. Leave to cool in the tin for 15 mins, then transfer to a wire rack to cool completely. Mix the icing sugar with 1 tbsp water, then drizzle all over to finish. Cut into 16 pieces to serve.
