How to Make It
Bring water, salt, and 1/4 cup of the butter to a boil in a small saucepan over medium-high. Reduce heat to medium-low; add flour, and stir vigorously with a wooden spoon until dough is smooth, about 30 seconds. Continue cooking, stirring constantly, until dough begins to pull away from sides of pan and a film forms on bottom of pan, 2 to 3 minutes. Transfer dough to a medium bowl. Stir constantly until slightly cooled, about 1 minute. Add eggs, 1 at a time, stirring constantly until completely smooth after each addition. Transfer mixture to a piping bag fitted with a medium star tip. Chill 30 minutes.
Pipe 6 (3-inch long) pieces in single layer in air fryer basket. Cook at 380°F until golden, about 10 minutes. Repeat with remaining dough.
Stir together sugar and cinnamon in a medium bowl. Brush cooked churros with remaining 2 tablespoons melted butter, and roll in sugar mixture to coat.
Place chocolate and cream in a small microwavable bowl. Microwave on HIGH until melted and smooth, about 30 seconds, stirring after 15 seconds. Stir in kefir. Serve churros with chocolate sauce.