Make the streusel by mixing all the ingredients together by hand until crumbly. Set aside in the refrigerator for at least 30 minutes.
Preheat the oven to 180°C (350°F/Gas 4). Beat the butter and sugar vigorously until light and creamy. Mix in the rest of the wet ingredients. Combine the dry ingredients and add to the wet mixture without overworking the batter.
Assembly and Cooking
Pour the batter to a depth of 1 cm (½ inch) into 14 buttered and floured round 12 cm (4½ inch) cake tins. Add a few pear slices and top with the streusel just out of the refrigerator (so the streusel holds together better during cooking), without packing it down too much. Bake for 30–35 minutes until a skewer inserted comes out clean.