Prepare the Donut Hole Batter:
In a large bowl, whisk together the flour, sugar, baking powder, salt, and cinnamon (if using).
In a separate bowl, whisk together the milk, egg, melted butter, vanilla extract, and pureed strawberries. If you're using strawberry jam, you can mix it in here as well.
Pour the wet ingredients into the dry ingredients and stir until just combined. Be careful not to overmix.
Heat the Oil:
Heat oil in a deep fryer or a large pot to 350°F (175°C). You need enough oil to submerge the donut holes.
If you don’t have a thermometer, you can test the oil by dropping in a small amount of batter—if it sizzles and rises to the top, the oil is ready.
Fry the Donut Holes:
Using a small spoon or cookie scoop, carefully drop spoonfuls of the donut hole batter into the hot oil. Don’t overcrowd the pan.
Fry the donut holes for 2-3 minutes, or until golden brown, turning occasionally to ensure they cook evenly.
Remove the donut holes from the oil using a slotted spoon and place them on a paper towel-lined plate to drain.
Coat with Sugar:
While the donut holes are still warm, combine the sugar and cinnamon in a shallow dish. Roll each donut hole in the sugar mixture until well coated.
Serve immediately while warm, or let them cool slightly before serving.