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Preheat your oven to 180°C (350°F) with both top and bottom heat, and place the paper liners in a standard muffin tin.
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Rub the lemon zest into the sugar using your fingertips.
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In a large bowl, mix together the neutral vegetable oil, sugar, vanilla extract, and a pinch of salt. Add the eggs and mix well until you get a smooth mixture.
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Stir in the yogurt until everything is well combined.
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In a separate bowl, mix the all-purpose flour, ground hazelnuts, cinnamon, and baking powder.
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Gently add the dry ingredients to the wet yogurt mixture and stir only until just combined. Be careful not to overmix the batter. Fold in 2/3 of the plum pieces.
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Fill the prepared muffin liners with the batter evenly and top with the remaining plum pieces. Sprinkle with some more cinnamon sugar for crunchy tops.
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Bake the muffins in the middle of the preheated oven for about 20-25 minutes or until golden brown. Don't forget to do the toothpick test.
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After baking, let the muffins cool briefly in the tin before transferring them to a wire rack to cool completely.