Heat a saucepan over high heat. Melt the butter and whisk in the brown sugar. Cook for 3 minutes. Pour into a small bowl and let cool for 5 minutes.

Prepare the dough. In a large bowl, combine the mochiko flour, tapioca flour, baking powder, salt, and granulated sugar. Make a well in the center and add the melted butter mixture and vanilla extract. Slowly add the hot water. Whisk thoroughly (preferably with a dough whisk) and cover with a damp kitchen towel. Let rise for 1 hour.

Fill a high-sided pot with about 3 or 4 inches of oil and heat it to 350°F.

Lightly flour a wooden cutting board. Remove the dough from the bowl and divide into 8 equal sections.

Roll each section into a ball, then divide each ball into 8 pieces. Roll each piece into a ball and arrange 8 pieces on a small square of parchment paper, forming a ring. Repeat with the remaining sections of dough.

Carefully add donut rings to the oil with the parchment paper to help hold their shape. Remove parchment paper after 2 minutes and flip the donuts. Cook for 1 more minute. Transfer the donuts to a wire rack set on baking sheet.

Prepare the glaze. Add the butter to a saucepan and melt it over medium-high heat. Once melted, stir in the brown sugar. Cook for 3 minutes, then remove from the heat and transfer to a small mixing bowl. Add the powdered sugar and milk to the bowl and whisk well

Dip the cooled donuts into the glaze and sprinkle with sea salt. Enjoy!