Step 1

Working in large bowl, mix the flour, baking powder, sugar and salt.

Step 2

Add the diced butter and the whole egg, then rub in with your fingertips until the mixture resembles fat breadcrumbs.

Step 3

Use half the crumbs to make a loose layer in the base of a buttered shallow 28cm tin and bake 170C (150C fan)/340F/gas 3½ for 10 minutes, until the crumbs are lightly golden.

Step 4

Remove and leave to cool.

Step 5

Put the milk in a pan and use a grater to take the zest off the lemons, letting it fall directly into the pan.

Step 6

Warm the milk gently over a very low flame, so it infuses.

Step 7

Meanwhile, in a bowl, whisk the egg yolks with the sugar and cornflour, until they form a thick paste. Then whisk in 150ml lemon juice.

Step 8

Once the milk is simmering, whisk the egg and lemon mixture into it and cook, stirring, until the mixtures thickens like custard.

Step 9

Spread the lemon cream over the crumb base, leaving a 1cm border around the edge, then cover with the remaining crumbs.

Step 10

Bake again for 20 minutes, until the crumbs are golden, then remove and leave to cool completely before inverting on to a plate.