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1
Preheat oven to 350 F. Spray an 8-inch cake pan (with at least 2-inch sides) with nonstick cooking spray with flour.
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2
To make the cake: In a large bowl, whisk together the sugar, flour, cornstarch, cocoa powder, baking powder, espresso powder, baking soda and salt. Add the eggs, canola oil and vanilla and whisk to combine. The mixture will be very thick - just make it as uniform as you can. Add the water, a little at a time, whisking in between each addition. Continue until you've added all of the water - the batter should be smooth and fairly thin.
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3
Transfer the batter to the prepared pan. Bake for 35-37 minutes, or until a toothpick inserted in the center comes out clean and the cake springs back when gently pressed with your fingertip. Transfer the pan to a wire rack and cool for 15 minutes, then turn the cake out onto the rack to cool completely.
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4
To make the filling: In the bowl of a stand mixer fitted with the paddle attachment, beat the peanut butter, sugar and vanilla together until the mixture resembles coarse crumbs. Add the cream, beating until the filling is smooth - it will be somewhat thick, but still spreadable. You can add additional cream to adjust the consistency, if necessary.
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5
To make the glaze: Combine the chocolate and cream in a microwave-safe bowl, and heat for 30 seconds on 50% power. Gently stir, then continue heating in 20-second intervals on 50% power, stirring in between each, until the chocolate is completely melted and the glaze is smooth. Set aside for 30-45 minutes, or until the mixture thickens enough to be spreadable.
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6
To assemble: Use a long, thin knife to cut the cake in half horizontally to make two layers. Place the bottom, cut side up, on a serving plate. Spread with the peanut butter filling, then lay the second half of the cake on top, cut side down. Spread the glaze over the top and sides of the cake. Garnish with chopped peanut butter cups.
