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1
In a stand mixer, add the cake mix, egg whites, vegetable oil, and the strawberry daquiri concentrate.
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2
Mix on low for about 30 seconds and then increase to medium speed for 90 seconds (the batter will still be a bit lumpy; take care not to overmix so you dont end up with dry cupcakes).
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3
Prepare muffin pan with cupcake liners (fill the cups between 2/3 and 3/4 full).
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4
Bake at 350 for about 20 minutes or until a toothpick registers done in the center cupcake.
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5
Remove the pan from the oven and let rest 5 minutes.
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6
Remove the cupcakes and cool completely on a baking rack.
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7
Once the cupcakes have cooled, frost the cupcakes.
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8
(Chill the cupcakes if planning to frost them later).
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9
FROSTING: First wash and hull strawberries and toss them into a food processor (I used a hand blender).
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10
Puree strawberries, put over a strainer to drain some of the liquid.
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11
Stir occasionally until most of the liquid has drained.
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12
Add sugar and egg whites into a mixer bowl or another large heatproof bowl, fit the bowl over a plan of simmering water and whisk constantly, keeping the mixture over the heat, until it feels hot to the touch, about 3 minutes.
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13
The sugar should be dissolved, and the mixture will look like shiny marshmallow cream.
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14
Remove the bowl from the heat.
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15
Using the whisk attachment on a stand mixer or with a hand mixer, beat the meringue on medium speed until it is cool, about 5 minutes.
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16
Switch to the paddle attachment if you have one (totally optional), and add the butter a tablespoon at a time, beating until smooth.
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17
Once all the butter is in, beat the frosting on medium-high speed until it is thick and very smooth, 6-10 minutes.
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18
Don't freak out if your frosting curdles or separates, keep beating and it will come together again.
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19
Once the frosting is thick and smooth, gradually beat in the lemon juice on medium speed, waiting until each addition is absorbed before adding more, and then the vanilla.
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20
By this point you should have a shiny smooth, velvety, pristine white frosting.
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21
You can stop at this point if you want to stick with lemon frosting.
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22
For strawberry buttercream, gradually add in strawberry puree, beating on medium speed until well incorporated.
