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1
Combine first 5 ingredients in heavy medium saucepan.
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2
Stir over low heat until both sugars dissolve.
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3
Increase heat and boil until large bubbles break on surface and sauce drops thickly from spoon, stirring constantly, about 2 minutes.
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4
Remove from heat; whisk in cream and vanilla (mixture may bubble vigorously).
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5
(Can be made 3 days ahead.
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6
Cover; chill.
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7
Rewarm slightly over low heat before using.)
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8
Preheat oven to 350F.
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9
Line two 9-inch-diameter cake pans with 1 1/2-inch-high sides with parchment paper; butter paper.
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10
Sift flour, baking soda and salt into small bowl.
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11
Using electric mixer, beat butter and both sugars in large bowl until well blended.
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12
Add egg and vanilla and beat until smooth.
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13
Beat in flour mixture.
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14
Stir in chocolate chips.
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15
Drop half of dough by large spoonfuls into each prepared pan; spread evenly.
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16
Bake cookies until light golden, about 18 minutes.
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17
Cool cookies in pans on racks.
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18
Carefully turn out cookies; peel off parchment.
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19
Place 1 cookie, top side up, in 9-inch springform pan with 2 3/4-inch-high sides.
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20
Drizzle 1/4 cup sauce over.
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21
Spread evenly with ice cream; drizzle with 1/2 cup sauce.
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22
Top with remaining cookie, top side up; press gently to adhere.
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23
Drizzle with 2 tablespoons sauce.
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24
Cover pan and freeze ice cream cake at least 5 hours.
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25
Cover and chill remaining sauce.
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26
(Cake and sauce can be prepared 3 days ahead.)
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27
Cut around cake with small knife to loosen.
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28
Release pan sides.
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29
Let cake stand until ice cream is slightly softened, about 20 minutes.
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30
Rewarm butterscotch sauce over low heat.
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31
Cut cake into wedges; serve with sauce.
