Delicious Apple Tartlets with Creamy Filling

Ingredients

• 100 grams (0.22 pound) unsalted butter, softened
• 25 grams (0.06 pound) sugar
• 1 egg yolk
• 50 grams (0.11 pound) sour cream (preferably 20% fat)
• 1 teaspoon baking powder
• 225 grams (0.5 pound) all-purpose flour
• a pinch of salt
• 200 grams (0.44 pound) condensed milk
• 1 egg
• 1 tablespoon cornstarch
• zest of half a lemon (optional)
• 2 medium apples
• cinnamon (optional)