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.)
Once the dough has doubled in size, punch it down to remove excess air, then transfer it to a clean, lightly floured surface.
Roll out the dough until it is ½" (~1 ¼ cm.) thick. Use a 3" heart cookie cutter to cut out the donuts, then re-roll the dough to cut out more donuts. You should end up with 6 but you may end up with more or less based on your cookie cutter.
Cut out 6 parchment squares that are ~4" each and place them on to a lined baking tray.
Transfer the hearts to their own parchment square, 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.
Spread the 150g of sugar out in to a shallow bowl.
Once the donuts 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 with a metal slotted spoon and immediately toss them in sugar (carefully!)