Add honey, water, and rhubarb to a small saucepan. Bring to a simmer, then turn heat to low for 10 minutes, stirring occasionally. Once honey has dissolved and rhubarb has started break down, remove from heat and allow to cool.
Once cool, strain into an airtight container and store in the refrigerator.
For the Rhubarb Bourbon Sour:
Add all ingredients to a shaker filled with ice. Shake.
Strain into a rocks glass filled with ice.
Add a piece of rhubarb as a garnish if desired.
Notes
If you are sensitive to gluten-grain-based bourbon, Hudson Baby Bourbon is a 100% corn-based and could be a good alternative.
Use extra stalks or make a rhubarb ribbon as a garnish. You can also add a lemon wheel.
Store the extra simple syrup in an airtight, sterilized container in the refrigerator for up to one week.