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1
Preheat the oven to 325F and set the rack in the middle.
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2
Spray an 8-inch springform pan with oil, line the bottom with baking parchment and then set it aside on a baking sheet.
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3
Put the sugar, butter and yogurt into a large bowl and beat like mad with a whisk until well combined.
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4
An electric mixer would be great for this job, otherwise get your whisk, roll up your sleeves and just go for it.
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5
This mix is not going to go all light and fluffy and pretty like its full-fat cousin, but you are just looking for some semblance of uniformity (it will have quite a few little lumps in, but that is okay).
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6
Next, add the whole eggs and half of the flour and beat for a good few minutes, until the mixture is well blended and starts to look a little smoother.
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7
Whisk the egg whites until light and frothy, then fold them into the mixture with the rest of the flour, the baking powder, vanilla extract and lemon zest and beat until you have a smooth batter.
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8
Finally, stir half of the blueberries through.
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9
Pour the mixture into the pan and smooth it down evenly with the back of a spoon.
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10
Arrange the remaining blueberries, flat side down, on the top in a few lines across the center.
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11
Pop into the oven for about 2530 minutes, until a knife inserted into the center comes out clean.
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12
The top should be quite a light golden brown and the sponge will feel firm to the touch.
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13
Meanwhile, to make the topping, put the confectioners sugar into a small bowl.
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14
Stir in the limoncello (or lemon juice) to make a smooth, runny mixture and set aside until ready to use.
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15
Once the cake is cool, place it on a serving platter or cake stand.
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16
Slowly spoon the topping all over the sponge and allow it to pour over the edges beautifully.
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17
Cut into 12 wedges and serve!
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18
If making in advance, keep it in the fridge.
