How can I flambé ice cream?
For a substance to burn, it must first reach its ignition point. For it to keep burning, it must reach its fire point. The ignition point of a 40% ABV liquid such as brandy is 26ºC/79ºF, and the fire point is approximately 10ºC higher than that.

What this means in practical terms is that you need to heat the alcohol a little first before you add it to the ice cream. The simplest way to do this is to put the alcohol in a ladle and heat it over the hob - it won't need much as 36ºC is not a great deal over room temperature. Then light it carefully with a long match, and pour it around your ice cream - if you've got it warm enough it should keep burning until the alcohol is consumed.