Preheat water bath using immersion circulator to 135 degrees F.
Use a vegetable peeler to remove the rinds from the oranges, peeling off as little of the white rind as possible.
Wash the rhubarb stalks thoroughly and chop into large chunks, discarding ends.
Add the vodka, orange rind, and rhubarb to a quart sized jar. Place in water bath and cook for 2 hours.
While the vodka mixture is cooking, make simple syrup. Juice the oranges into a large liquid measuring cup. After juicing, add water until total liquid is 2 cups. Combine liquid and sugar in a saucepan over low heat. Cook until sugar is dissolved, about 15 minutes, stirring occasionally. Let cool to room temperature.
When vodka mixture is done, strain mixture into a large bowl and discard orange peels. Add simple syrup and whisk to combine.
Taste the mixture. Add bitters a few dashes at a time, stir, and taste. Repeat until the mixture has reached your desired bitterness. Don't be afraid to lean to the bitter side!
Store in a cool dark place or in your refrigerator.
To make a homemade Aperol spritz, add 2 oz of the homemade aperol to a large wine glass. Add a large handful of ice. Top with prosecco and garnish with an orange slice.