Fill the mugs: Spray the insides of two 12-ounce mugs (1 1/2 cups capacity) with cooking spray. In a medium bowl, add pineapple and juice along with the brown sugar. Stir until brown sugar is dissolved as much as possible. Divide in half and pour into mugs.
Top the mugs: Wipe out the same medium bowl with a paper towel (less dishes!) and add egg, oil, baking powder, pumpkin pie spice, sugar, flour and milk, in that order. Whisk between each addition to combine. Pour half into each mug.
Zap the mugs! Place both mugs in the microwave and zap for 4-5 minutes. Remove and allow mugs to cool slightly before eating. Extra credit if you add a scoop of ice cream! Eat from the mug or cover the mug with a plate facing down, then flip and allow the cake to rest on the plate with the pineapple bottom on top!
Zapping time: 4-5 minutes