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1
Preheat oven to 350 F. Line two 12-cup muffin pans with paper liners.
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2
Whisk both flours, the baking powder and baking soda together in a medium bowl. In the bowl of a stand mixer fitted with the paddle attachment, cream the butter, sugar and salt on medium speed until light and fluffy, about 2-3 minutes. With the mixer on low, gradually add the dulce de leche, continuing to beat until completely incorporated. Add the eggs one at a time, beating well after each addition. Add the oil and vanilla. With the mixer on low, alternately add the dry ingredients (in 3 additions) and the buttermilk (in 2 additions), beginning and ending with the dry ingredients. Mix just until incorporated.
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3
Divide the batter among the prepared pans, filling each well about 2/3 - 3/4 full. Bake the cupcakes for 18-22 minutes, or until the tops spring back when lightly pressed and a toothpick inserted in the center comes out clean. Transfer the pan to a wire rack and let the cupcakes cool for about 5 minutes before removing them to the rack to cool completely.
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4
To make the buttercream: In the bowl of a stand mixer, whisk the egg whites, sugar and salt together. Set the bowl over a pan of simmering water and heat, whisking constantly, until the mixture registers 160 F on an instant read thermometer.
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5
Secure the bowl on the mixer and fit the mixer with the whisk attachment. Beat the mixture on medium-high speed until stiff peaks form, and the outside of the mixer bowl is cool to the touch, about 7-8 minutes. With the mixer on medium speed, gradually add the butter, 2 tablespoons at a time, waiting until each piece is incorporated before adding the next. Increase the mixer speed to medium-high and beat until the frosting is thick and smooth, 3-4 minutes. Mix in the dulce de leche and vanilla and beat until incorporated.
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6
Place the frosting in a pastry bag fitted with a decorative tip. Frost the cupcakes, then garnish as desired.
