Lemon Brioche Donuts

Light and fluffy brioche lemon donuts, filled with lemon mascarpone whipped cream & lemon curd and coated in lemon sugar.

In a microwave safe measuring cup, heat the milk to 100°F / 38°C. Whisk in the yeast and the sugar. Set aside until the mixture begins to foam. (~5 minutes)
While waiting, add the flour, salt, egg, and butter to the bowl of a stand mixer fitted with the dough hook attachment. When the yeast mixture begins to foam, add it in to the rest of the ingredients.
Knead the dough on low speed for 1 minute to allow the ingredients to come together. Increase the speed to medium-high and knead for an additional 10-20 minutes, or until the dough passes the windowpane test. (This will happen after the dough has cleaned the sides and bottom of the bowl)
Once the dough is done kneading, butter the inside of the bowl, form the dough into a ball and place it back in the bowl. Then cover the mixing bowl with plastic wrap. At this point, you could set the dough in the fridge to rise overnight, or continue with completing the recipe in one day. If doing in one day, set the dough in a warm area and wait for it to double in size (~1 hour.)

While the dough is rising, make the lemon curd.
Add the egg yolks, sugar, lemon juice, lemon zest, and salt to a small saucepan.
Whisk the mixture vigorously over medium heat until it thickens, and reaches at least 170°F / 76°C.
Remove the curd from the heat and pass it through a fine mesh sieve to remove the zest and any accidental cooked egg.
Immediately mix in the butter until it melts and the mixture is completely smooth.
Transfer the lemon curd to an airtight container and store it in the fridge until you're ready to fill the donuts.

Once the dough has doubled in size, punch it down to remove excess air, then transfer it to a clean, lightly floured surface.
Divide the dough into 6 equally sized pieces, and roll each tightly into a ball.
Cut out 6 parchment squares that are ~4" each and place them on to a lined baking tray.
Transfer the dough balls each to their own parchment square. Press down on each of the balls to slightly flatten them, then cover them loosely with a towel.
Allow them to rise in a warm environment for ~30 minutes, or until they have slightly puffed up.
Make the lemon sugar coating: add the sugar and lemon zest to a bowl. Use your fingers to rub the lemon zest into the sugar, and set aside.
Once the buns have puffed up, fill a large saucepan with at least 2" of vegetable or any neutral oil. Heat the mixture to 365°F / 185°C.
Carefully place the donuts in to the fryer with the parchment paper (2-3 at a time, depending on what your pan can fit). After ~5-10 seconds, remove carefully remove the parchment from the donuts using metal tongs.
Fry the donuts on each side for 3-4 minutes, or until they are a deep golden brown. You can also check if they are done by making sure the internal temperature is at least 190°F / 87°C.
Remove the donuts from the oil and immediately toss them in the lemon sugar.

In a small mixing bowl, beat together the mascarpone, lemon zest, sugar, and lemon extract (if using) until just combined.
While beating the mixture, slowly stream in the heavy whipping cream. Continue beating until soft peaks form.
Once the donuts have cooled to room temperature, use a sharp knife to poke a hole in the top of each donuts.
Transfer the whipped cream into a piping bag fitted with a round tip, leaving room on one side of the piping bag to put in the lemon curd. Pour the lemon curd into the bag as well. Fill each donut generously with the whipped cream & lemon curd, then serve and enjoy!