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1
Preheat oven to 350F.
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2
Toast nuts in a shallow baking pan in middle of oven until skins are split and nuts are golden, 10 to 12 minutes.
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3
Wrap nuts in a kitchen towel and rub to remove skins (not all skins will come off), then cool.
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4
Finely chop 1/3 cup nuts and reserve.
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5
Finely grind remaining 1/3 cup whole nuts with cornstarch and 1 teaspoon sugar in a food processor (do not allow to become a paste).
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6
Transfer to a bowl and stir in 1/4 cup sugar.
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7
Reduce oven to 250F.
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8
Cover 2 baking sheets with foil.
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9
Using cutter or cardboard template as a guide, trace 6 hearts on each sheet with pointed end of a toothpick (for a total of 12).
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10
Brush foil lightly with oil.
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11
Beat egg whites and a pinch of salt with an electric mixer until foamy, then add cream of tartar and continue to beat until whites hold soft peaks.
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12
Gradually add 1/2 cup sugar, beating until whites just hold stiff, glossy peaks.
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13
Fold in ground-nut mixture gently but thoroughly and transfer meringue to pastry bag.
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14
Staying within traced lines, pipe 4 solid hearts and 2 hearts with a heart-shaped space in center on each baking sheet (there may be meringue left over).
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15
Bake in upper and lower thirds of oven, switching position of sheets halfway through baking, until dry and pale golden, 1 to 1 1/4 hours.
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16
Cool 10 minutes, then carefully peel meringues off foil and transfer to a rack to cool completely.
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17
Stir together espresso powder and 1 teaspoon warm water until dissolved.
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18
Bring remaining 1/4 cup warm water and remaining 1/2 cup sugar to a boil in a very small heavy saucepan over moderate heat, covered, stirring occasionally until sugar is dissolved.
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19
Boil, uncovered, washing down sugar crystals from side of pan with a pastry brush dipped in water, until candy thermometer registers 238F or a drop of syrup forms a soft ball when dropped into a cup of cold water.
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20
When syrup reaches boil, start beating yolks in a bowl with electric mixer until thick and pale.
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21
When syrup reaches soft-ball stage, slowly pour it in a thin stream down side of bowl into yolks, beating constantly.
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22
Continue to beat at high speed until cool to the touch.
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23
(It is important that mixture is properly cooled before proceeding.)
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24
With mixer at high speed, add butter 1 piece at a time, beating well after each addition until incorporated, then beat in dissolved espresso.
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25
Continue to beat buttercream until fluffy.
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26
Spread 1 tablespoon buttercream evenly over each solid heart.
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27
Top each of 4 solid hearts with another solid heart, buttercream sides up, then top each stack with an open heart.
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28
Spread about 1 tablespoon buttercream around sides of each heart, then coat sides with reserved chopped hazelnuts.
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29
Chill hearts 30 minutes, then let stand at room temperature 15 minutes before serving with coulis.
