Subtle House's Cookbook

5 rounded Tbsp cake mix
1 Tbsp neutral oil
2 Tbsp water
up to 2 Tbsp add-ins (peanut butter, frozen raspberries, chocolate chips, jam, etc.)

or

6 rounded Tbsp cake mix
1 Tbsp unsweetened applesauce or mashed banana
2 Tbsp water
up to 2 Tbsp add-ins (peanut butter, frozen raspberries, chocolate chips, jam, etc.)

or (for Simple Mills almond flour brownie mix specifically)

46 mg cake mix (about 5 Tbsp probably)
1 Tbsp unsweetened applesauce
1 Tbsp vegetable oil
1 tsp water

Measuring with a soup spoon is fine. It's mug cake, not science.

In a large mug or two-cup ramekin, mix all ingredients except the add-ins. Mix in add-ins, or make a well in the center of the batter and drop add-ins in there.

Microwave at 80% power for 1.5 to 2 minutes (shorter if you used the first or third recipe, longer if you used the second recipe or lots of add-ins) or until you just start to smell warm cake. If your microwave is lower-wattage, full power is probably fine. If you smell anything like scorching, remove from the microwave immediately.

This is a little elaborate for a mug cake, but you do end up with a cake in a mug, so whatever.

Genius Kitchen (untested)

Put ice cream on counter to soften.

1 tsp butter (dairy or non-dairy)
1/2 beaten egg (doesn't have to be exact, just whisk it up and then discard about half)
1/8 tsp vanilla

Melt butter in mug. Let cool a bit, then whisk in egg and vanilla.

In four-cup microwave-safe measuring pitcher, combine:

1 Tbsp unsulphured blackstrap molasses
1/2 C milk (dairy or non-dairy)
1 Tbsp sugar
1 Tbsp cornmeal
1/4 tsp cinnamon
1/8 tsp nutmeg
1/8 tsp ground ginger

Microwave on high for two minutes. Stir gently. Microwave on high for one minute at a time until mixture thickens.

Pour a tiny bit of the molasses mixture into the egg mixture, whisking rapidly, to warm it up without cooking the egg. Continue slowly adding molasses mixture to egg mixture and whisking until it's all combined in the mug.

Microwave at 50% power for one minute at a time until just cooked through, with a soft cake-like texture.

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Alternative method (also untested): Omit egg, combine all other ingredients in mug, and microwave at 50% power for one-minute intervals, stirring in between, until pudding-like and delicious.

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Durgin-Park recipe (untested):

Put ice cream on counter to soften.

2 Tbsp granulated yellow cornmeal
1 Tbsp black molasses
1.5 tsp granulated sugar
1.5 tsp unsalted butter
pinch kosher salt
pinch baking soda
1/4 whisked egg
3/4 cup warmed whole milk, divided

Combine the first seven ingredients and half the milk. Microwave on high for two minutes. Stir gently. Microwave on high for one minute at a time until mixture thickens.

Whisk in the rest of the milk and microwave at 50% power for one-minute intervals, stirring in between, until pudding-like and delicious.

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Joy of Cooking recipe (tested):

Put ice cream on counter to soften.

5/8 cup milk
2 tsp cornmeal (not Tbsp!)
1.5 tsp butter
1 Tbsp molasses
1/8 tsp salt
1/8 tsp ginger
1 tsp sugar
optional: 1 pinch cinnamon
optional: 1 Tbsp raisins or 0.25 cup thinly sliced apples.

Put milk and butter in mug and microwave on high one minute or until boiling. Stir in cornmeal. Microwave on 50% power for 30 seconds at a time (or as long as you can go without it boiling over), stirring gently in between, until mixture thickens enough that it stops threatening to overflow.

Whisk in the rest of the ingredients and microwave on 50% for one-minute intervals, stirring in between, until cornmeal is soft and pudding is pudding-like and delicious. This will take a while, but don't increase the power or rush it; just keep stirring.

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Let cool slightly. Serve with whipped cream or vanilla ice cream.

Sources:
http://www.geniuskitchen.com/recipe/indian-pudding-microwave-51013
https://newengland.com/today/food/old-fashioned-durgin-park-indian-pudding/
The Joy of Cooking