In a mixing bowl, combine water, sugar, butter, egg yolks, yeast, salt, and vanilla extract until blended.
In a separate bowl, whisk flour and baking powder, then gradually add to the mixture until a soft dough forms.
Knead the dough for 5–10 minutes until smooth and slightly sticky.
Transfer to a greased bowl, cover, and let rise for about 1 hour or until doubled in size.
Roll the dough into a rectangle and cut into bar shapes.
Place on lined baking sheets, cover, and let rise again for 1 hour.
Heat oil to 350°F (180°C) and fry donuts for about 1 minute per side until golden brown.
Let donuts cool on a wire rack.
Mix all glaze ingredients until smooth.
Dip each donut into the glaze and let set before serving.
These homemade maple bar donuts are best enjoyed fresh while soft and slightly warm.