Chocolate Brownie advise please

[HaveToWearHeels]

Entertaining next weekend for DH's birthday. Want to do as little on the evening as possible. DH wants warm chocolate brownie with ice cream for dessert, what is the best way of achieving this ?
Baking in the afternoon then microwave it to warm it or prepare mix and pop in the oven just before people arrive, but may not have room in the oven ?
Or any other suggestions ?

Warm up in microwave. Short blast so not too hot. Cut into squares first.

Thanks Selvedge, I thought that would be OK.

Have to wear heels - I have PMed you my favourite brownies recipe for your consideration!

1/4 cup sugar
1/4 cup flour
2 tbsp good quality (read G&B...) Cocoa powder
Pinch of salt
2 tbsp olive oil
3 tbsp fizzy spring water

Put all dry ingredients in a mug and stir. Add wet ingredients and stir. Microwave for 1min40s. Enjoy. (Under 100 calories but tastes full fat...)