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1
Preheat the oven to 350F.
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2
Line two standard 12-cup muffin pans with paper liners.
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3
Into a large bowl, sift together flour, baking powder, salt, and cinnamon; set aside.
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4
In the bowl of an electric mixer fitted with the paddle attachment, beat the butter and sugar on medium speed until creamy, about 3 minutes.
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5
Add the eggs and vanilla, and beat until combined.
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6
With the mixer still on medium speed, add the flour mixture in two parts, alternating with the milk and beginning and ending with the flour.
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7
Stir in the walnuts with a wooden spoon.
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8
Divide the batter evenly among the muffin cups, adding about 1/3 cup to each.
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9
Bake, rotating pans halfway through, until cupcakes are golden and a cake tester inserted in the center of a cupcake comes out clean, 18 to 20 minutes.
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10
Transfer the pans to a wire rack to cool slightly.
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11
Invert cupcakes onto the rack; then reinvert and let them cool completely, top sides up.
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12
Frost tops with Maple Buttercream and garnish with Candied Walnuts, if using.
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13
Cupcakes can be refrigerated in an airtight container for up to 3 days.
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14
In the bowl of an electric mixer fitted with the whisk attachment, beat the egg yolks on high speed until light and fluffy, about 5 minutes; set aside.
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15
In a small saucepan set over medium-high heat, bring the maple syrup to a boil, and cook until it registers 240F on a candy thermometer, about 15 minutes.
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16
Remove from heat.
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17
With the mixer running, slowly pour syrup down the side of the bowl in a slow, steady stream, until completely incorporated, about 1 minute.
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18
Continue beating until bowl is just slightly warm to the touch, 4 to 5 minutes.
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19
Add butter, one piece at a time, until thoroughly incorporated and the frosting is fluffy, about 4 minutes more.
