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1
Chill a small metal baking pan in freezer. Form 18 ice cream balls with scoop and freeze in chilled pan at least 1 hour (this will make serving faster).
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2
Preheat oven to 425u00b0F with rack in middle. Butter a large baking sheet.
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3
Bring butter, water, and salt to a boil in a small heavy saucepan, stirring until butter is melted. Reduce heat to medium, then add flour all at once and cook, beating with a wooden spoon, until mixture pulls away from side of pan and forms a ball, about 30 seconds. Transfer mixture to a bowl and cool slightly, 2 to 3 minutes.
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4
Add eggs 1 at a time, beating well with an electric mixer after each addition.
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5
Transfer warm mixture to pastry bag and pipe 18 mounds (about 1 1/4 inches wide and 1 inch high) 1 inch apart on baking sheet.
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6
Bake until puffed and golden brown, 20 to 25 minutes total. Prick each profiterole once with a skewer, then return to oven to dry, propping oven door slightly ajar, 3 minutes. Cool on sheet on a rack.
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7
Heat sugar in a 2-quart heavy saucepan over medium heat, stirring with a fork to heat sugar evenly, until it starts to melt, then stop stirring and cook, swirling pan occasionally so sugar melts evenly, until it is dark amber.
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8
Remove from heat, then add cream and a pinch of salt (mixture will bubble and steam). Return to heat and cook, stirring, until caramel has dissolved.
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9
Remove from heat and add chocolate, whisking until melted, then whisk in vanilla and Cognac (if using). Keep warm, covered.
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10
Halve profiteroles horizontally, then fill each with a ball of ice cream. Put 3 profiteroles on each plate and drizzle generously with warm chocolate sauce.
