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1
Dissolve cornstarch in lemon juice.
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2
Whisk egg yolks, sugar, 4 tablespoons butter and lemon peel in heavy saucepan.
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3
Whisk in cornstarch.
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4
Whisk over low heat until smooth and thick enough to mound in spoon, about 5 minutes; do NOT boil.
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5
Cool to room temperature, whisking occasionally.
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6
Add remaining 6 tablespoons butter 1 piece at a time, whisking until smooth.
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7
Cover tightly and refrigerate until well chilled.
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8
This part can beprepared 2 days ahead.
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9
Position rack in center of oven and preheat to 350F (180C).
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10
Dab butter in 3 places on rimless baking sheets or use rimmed sheets turned over and dab the butter on the bottom side.
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11
Line with foil, leaving 2-inch overhang on short ends.
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12
Combine sugar, oats, flour and baking powder in large bowl.
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13
Add melted butter, milk, molasses and vanilla and stir with fork until just blended.
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14
Let stand 15 minutes.
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15
Drop batter onto prepared sheets by heaping teaspoonfuls, forming into 1-inch diameter rounds and spacing 3 inches apart.
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16
Bake until cookies bubble and begin to brown around edges, 6 to 7 minutes.
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17
Transfer foil with cookies to a work surface.
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18
Flatten cookies gently with spatula if necessary.
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19
Cool completely.
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20
Cookies can be prepared 1 day ahead--store airtight.
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21
Just before serving, spread 3 teaspoons of filling evenly on bottom side of cookie, top with another cookie, bottom side in.
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22
Repeat procedure with remaining cookies and filling.
