Preheat your oven to 155°C / 135°F fan / gas mark 3. Lightly oil a 26 x 12cm loaf tin and line the base with baking parchment.
Spread the ground almonds and whole hazelnuts on separate baking trays. Toast in the preheated oven for 15–20 minutes, turning every 5 minutes, until fragrant and golden brown. The hazelnuts may take slightly longer.
Set aside to cool completely.
In a large mixing bowl, whisk together the eggs, oil, and muscovado sugar until well combined. You can also use a stand mixer fitted with the beater attachment.
Add the flour, toasted ground almonds, baking powder, ginger, and salt to the wet .
Mix until just combined, then fold in the grated parsnip.
Spoon the batter into your prepared loaf tin and smooth the top.
Bake for 45–50 minutes, or until a skewer inserted into the centre comes out clean.
Let the cake cool in the tin for 10 minutes before turning it out onto a wire rack to cool completely.
To make the icing, beat the softened butter and sifted icing sugar in a bowl until light and fluffy.
In a separate small bowl, mix the Horlicks powder with 2 tablespoons (30 ml / 1 fl oz) of boiling water to form a smooth paste.
Beat this malted paste into the buttercream until fully incorporated.
Once the cake is completely cool, spread the Horlicks icing generously over the top. Roughly chop the toasted hazelnuts and scatter them over the icing.
Finish with a light dusting of icing sugar, if desired, before slicing and serving.
Don't skip toasting the ground almonds and hazelnuts. This simple step deeply enhances their nutty flavour and is crucial for the final taste of the cake.
