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1
Place rack in center of oven and preheat to 400 degrees F.
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2
Lightly spray the 12 indentations of a standard muffin tin with nonstick spray, or line each indentation with a paper muffin cup; set aside.
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3
Whisk the flour, graham cracker crumbs, baking powder, and salt, if using, in a medium bowl; set aside as well.
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4
Beat the peanut butter, sugar, and butter together in a large bowl with an electric mixer at medium speed until smooth, about 1 minute.
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5
Beat in the egg, then the egg white, and finally the vanilla until smooth.
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6
Scrape down the sides of the bowl with a rubber spatula and beat in the milk until creamy and light, about 1 minute.
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7
Remove the beaters and stir in the prepared flour mixture with a wooden spoon or a rubber spatula, just until moistened. The batter may still be grainy but no white pockets of flour should be visible.
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8
Fill the muffin-tin indentations three-quarters full. Reserve any leftover batter for a second baking.
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9
Bake until golden and a toothpick inserted into one of the muffins comes out dry, about 20 minutes.
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10
Cool the muffins in the tin on a wire rack for 5 minutes, then pop the muffins out of the tin and cool completely on the rack.
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11
They can be stored in an airtight container for up to 3 days. Or place them in a zip-closed plastic bag and freeze for up to 4 months.
