Add yeast, warm water, and ½ teaspoon of sugar to bowl of a stand mixer. Stir and let sit until the yeast starts to foam. In a large measuring cup, add milk and microwave for 30 seconds. Add butter and stir together until butter has melted.
Add flour, ⅓ cup sugar, and salt and mix together with a dough hook. Add in egg and mix on low. With the mixer still on low, slowly drizzle in melted butter and milk mixture. Once it's all poured in, let the dough hook mix everything together until the dough forms.
Once dough forms, turn up speed to medium and knead dough for 8 minutes. If the dough is still sticking to the sides after 6 minutes, add flour a tablespoon at a time until it loosens up. Place dough ball in a greased bowl and cover with a towel. Let it rise for 1½ hours. It should be double in size.
Prepare 1 baking sheet by lining with parchment paper. Punch down risen dough and remove from bowl. Place on clean counter with a little sprinkled flour. Cut dough into 4 even slices and then cut each into 3 slices for a total of 12. Working with one piece at a time, roll into a 4-inch log. Lay on prepared baking sheet. Repeat with rest of the dough and set logs about ½ inch from each other. You'll have to arrange in two rows so that all the logs of dough fit on the sheet.
Cover buns with a damp towel and let them rise again for 30 minutes or until fluffy and plump. Preheat oven to 350 degrees. Remove towel from buns. Bake in preheated oven for 20 minutes, or until deep brown on top. Remove from oven and let cool completely. Slice down the center vertically but do not slice all the way through.
Add heavy cream and sugar to a clean bowl of a stand mixer. Using a whisk attachment, beat cream on medium high until stiff peaks form. Place into a piping bag with a large star tip. Place jam in piping bag with a round tip. Pipe a stripe of jam into bottom of slice buns. Then pipe whipped cream on top. Finish with another piping of jam on top. Dust the tops with powdered sugar.