This is my ideal cake. Lemony, soft-crumbed and streusel-topped. 
 

Method:

• Mix 80g plain flour, 20g porridge oats, 2 tablespoons caster sugar and a pinch of sea salt in a bowl.
• Add the zest of 1 unwaxed lemon, then add 70g of cold unsalted butter. Use your fingers to rub the butter into the flour like a crumble mixture until large sticky clumps have formed. This is your streusel.
• Preheat the oven to 180°C/160°C fan. Grease a 23cm cake tin with butter, then line with baking paper.
• Put 250g plain flour, 80g ground almonds, 1½ teaspoons baking powder and 1 teaspoon bicarbonate of soda into a mixing bowl with ½ teaspoon sea salt and mix with a whisk until there are no lumps.
• In a stand mixer with the paddle attachment, or in another mixing bowl with an electric hand whisk or wooden spoon, cream 200g unsalted butter and 250g golden caster sugar until pale and fluffy. (About 3-4 minutes in a stand mixer — longer by hand.)
• Scrape down the bowl and add 3 large organic or free-range eggs one at a time, mixing on a low speed until each one is incorporated, then mix in 1 teaspoon pure vanilla extract, the zest of another unwaxed lemon, and 250g natural yoghurt or oat yoghurt.
• Add the dry ingredients to the batter and mix until just combined. Minimal mixing will make the cake as light as possible.
• Scrape the batter into the prepared tin and level gently with a spatula, then spoon over 100g good-quality lemon curd in little patches using your spoon to swirl it in a little. Scatter the streusel topping evenly to the very edges of the cake; don't pile it into the middle or it will sink.
• Bake for 1 hour, or until a skewer inserted into the middle of the cake comes out clean. Cover the top with foil if it looks like it's browning coo fast.
• Cool for 15 minutes in the tin, then remove from the tin and leave to cool completely on a wire rack.
• Serve with creme fraiche rippled with a little lemon curd.

Adapted from Easy Wins (4th Estate)