Preheat the oven 180º celsius.

Butter a large baking dish.

Add the flour to a mixing bowl and make a well in the centre. Add the yeast and half of the milk as well as the 3 tablespoons of sugar. Mix the liquid ingredients with a spatula, incorporating just enough of the flour to make a wet paste. Let sit 15 minutes.

Stir in the remaining milk, egg yolk, 3 tablespoons of melted butter, salt, and lemon zest, and mix everything together well. Knead the dough on a lightly floured surface until smooth. Press the dough into the buttered baking dish making sure there is a slight lip all around the edges. Cover with a kitchen towel, and let rise in a warm place for 1 hour, until doubled.

While the dough is rising, make the topping. Put the ingredients all together in the bowl of a food processor and pulse until the mixture resembles breadcrumbs.

Place the plum halves really close together with skin side down all over the dough and within the rim.

Sprinkle the topping over the top of the plums and in all the gaps (there may be a bit leftover) and let the dough rise a further 20 minutes.

Bake the cake for approximately 25 minutes, or until both the topping and base are golden brown. Remove from the oven and let cool before slicing to serve.