In a large bowl, whisk together the flour, granulated sugar, baking powder, salt, and cinnamon.
In another bowl, combine the milk, melted butter, egg, and vanilla extract. Mix well.
Pour the wet ingredients into the dry ingredients and stir until just combined. Do not overmix.
Heat about 2 inches of vegetable oil in a deep pot over medium heat until it reaches 350°F.
While the oil is heating, prepare the filling by mixing the softened cream cheese, powdered sugar, diced strawberries, and lemon juice in a bowl until smooth.
Once the oil is hot, use a cookie scoop or tablespoon to drop the donut batter into the oil, frying a few at a time until golden brown, about 2-3 minutes per side.
Remove the donuts with a slotted spoon and drain on paper towels.
Allow the donuts to cool slightly, then use a piping bag or a small knife to fill each donut with the strawberry cheesecake mixture.
Dust the filled donuts with additional powdered sugar before serving.