Measure all dry flours (oat, millet, sweet rice) and cornstarch into a small bowl and whisk gently to combine. Set this bowl near the stove.

Fit a piping bag with a 1/2-inch closed star tip.

Place a wire rack over a baking sheet to drain finished churros.

Line a shallow bowl with the cinnamon sugar mixture for coating later.

Pour about 1½ inches of vegetable oil into a large saucepan and heat over medium until it reaches 360° to 365°F.

In a medium saucepan, combine water, butter, sugar, and salt. Bring to a gentle boil.

Once boiling, remove from heat and add the whisked flours all at once, stirring energetically until a smooth dough forms.

Transfer the dough to a bowl and let it cool for 10 minutes.

Add the egg and vanilla extract, beating until glossy and slightly sticky.

Scoop dough into the piping bag. Pipe 4–6 inch strips into the hot oil, working in small batches.

Fry each batch for 1½–2 minutes until golden brown.

Remove churros to a wire rack to cool briefly.

Toss each churro in the cinnamon sugar until fully coated. Serve warm.

Store churros uncovered at room temperature for up to 12 hours or in an airtight container for 2–3 days. Reheat in the oven to restore crispness.