what should i make with the last cup of sweetened hazelnut milk?

Pumpkin bisque soup. Use the hazelnut milk instead of half & half.

Crepes if you have eggs.

Use for making overnight oats or cooking your oats. Probably won’t need any additional sweetner for the oatmeal when eating it.
Could also use for chia pudding

Probably too late now, but...
Rice pudding. (This is basically Mark Bittman's recipe.)

4 cups milk, 1/2 cup rice, 1/4 cup sugar, punch salt. Put in a 6 cup casserole dish, stir to ensure everything is combined (no sugar clumps). Bake with no lid at 300degrees F for 90 minute, stirring every 30 minutes. You'll get a skin on top; stir it into the pudding. It may need another 10 mins cooking, but watch it because it will thicken up as it cooks.

You can use use dairy/coconut milk/nut milks/canned evaporated singly or in any combinations. Long or short grain rice, white or brown; basmati, arborio -- I've used them all. You can stir in raisins at the 60 minutes mark. Add a splash of vanilla when you take it out of the oven, or rosewater, or orange flower water. Sprinkle with cinnamon, nutmeg, cardamom, apple pie spice, pumpkin pie spice, what have you.

Maybe make really dark hot chocolate, then lighten with the hazelnut milk? Can't beat hazelnut-chocolate combo. A shot of bourbon, rum or brandy might be nice in that cocoa at cocktail hour... I recently made hazelnut praline that I've been grinding and adding to whipped cream (for a chocolate mousse) and pastry cream (for cream puffs). Works!