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Preheat oven to 325 degrees F. Line the bottom of a 9 inch spring form pan with parchment paper but do NOT grease the sides. Greasing the sides will not allow the cake to rise well and since the cake is cooled in the pan, the ungreased sides provide support for the cake as it cools so that it will not shrink.
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In the bowl of an electric mixer, using a whisk attachment, combine the sugar, eggs and vanilla extract and beat on medium high speed for about 10 minutes until the mixture is foamy and pale in colour.
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Reduce the speed of the mixer to medium low and with the mixer running, begin to continuously sprinkle in rounded tablespoons of the flour. Stop the mixer as soon as the flour is fully incorporated.
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Remove 1 to 2 cups of the sponge cake batter and mix it with the melted butter.
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Immediately add this mixture back into the main batter, folding it in very gently with a rubber spatula. Make sure the butter mixture is fully mixed in but be careful not to over mix the batter when folding as this can cause the batter to deflate.
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Pour the batter into the prepared 9 inch spring form pan and bake for approximately 45 minutes or until the top springs back fully when pressed lightly. Watch this cake closely as it will over bake easily if left for 5 minutes too long. Start checking it at the 40 minute mark just to be sure, although it always takes the full 45 minutes in my oven.
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Cool the cake completely IN THE PAN for at least a couple of hours before carefully and slowly running a sharp knife around the outside edge of the pan to release the cake from the sides.
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Release the sides of the spring form pan and peel the parchment paper off the bottom of the cake.
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Using a sharp serrated knife, cut the cake into small cubes.