School cake with custard

Grease a square pan and lay in a parchment strip cut to the width of the bottom, long ends up as a sling; crease them back so they do not flop into the batter.

Beat the softened butter with the granulated sugar on high speed for a few minutes until fluffy, like whipped butter.

Beat in the eggs one at a time, then the vanilla.

Add the baking powder and a pinch of salt, then 200 grams of flour, and bring together until just smooth; overmixing makes the cake tough. It should look more like cookie dough than cake batter.

Spread out to the pan edges and bake at 350F, 180C; 325F with a convection fan on.

For the custard, whisk the egg yolks, sugar, a little vanilla, a pinch of salt and the cornstarch with just enough of the dairy to make a thick, smooth paste while everything is cold.

Put the pan over direct heat, add the rest of the dairy and whisk constantly; as it reaches a simmer the whisk drags and the custard thickens. Pull it off the heat at once. No tempering and no double boiler.

Pour into a cold container, leaving the thick stuff on the pan sides behind; chill, speeding it up in a bowl of ice water with a stir every few minutes. Thin with more cream if it sets too thick.

Test the cake at half an hour and pull it as soon as a toothpick comes out with little or no crumb; this cake dries out easily.

Cool a minute, lift out with the parchment sling, flop it over, peel off the paper and cool completely on a rack before icing.

Mix the powdered sugar with the water, starting with two tablespoons and adding just enough to make a thick fluid icing, barely a liquid.

Scrape it all onto the cake and spread gently without force so the cake does not tear.

Scatter rainbow sprinkles right away while the icing is wet, then let it dry before slicing into squares.