In a large bowl, whisk together the flour, sugar, baking powder, cinnamon, and salt. In another bowl, mix the milk, eggs, vanilla extract, and vegetable oil. Gradually add the wet ingredients to the dry ingredients, stirring until just combined.

Shape the Doughnuts

Transfer the batter to a piping bag and pipe into hot oil in a circular motion to form doughnuts. Fry until golden brown on both sides, about 2-3 minutes per side.

Drain and Cool

Remove the doughnuts from the oil and place them on paper towels to drain excess oil. Allow them to cool slightly.

Coating and Filling

Cinnamon Sugar Coating

In a shallow bowl, mix 0.5 cup granulated sugar with 1 tsp ground cinnamon. Roll the warm doughnuts in the cinnamon sugar mixture until fully coated.

Fill the Doughnuts

Use a pastry bag to fill half of the doughnuts with dulce de leche, the other half with raspberry jam, or drizzle with chocolate ganache.