Step 1: Preheat the oven to 350°F. Grease a 9-inch tart pan with removable bottom.
Step 2: In a medium bowl, combine flour, salt, and baking powder. Add softened butter and sugar, and mix well with a pastry cutter or your hands until crumbly.
Step 3: Press the mixture into the bottom of the greased tart pan, and bake for 10-12 minutes, or until lightly golden brown. Allow to cool completely.
Step 4: In another medium bowl, whisk together eggs, vanilla extract, milk, granulated sugar, brown sugar, melted butter, cornstarch, and lemon juice until well combined.
Step 5: Pour the mixture over the cooled tart bottom, and bake for an additional 25-30 minutes, or until the filling is set and the top is golden brown. Allow to cool completely.
Step 6: Sprinkle powdered sugar on top, if desired. Slice and serve!
Enjoy these delicious Grandma's Sand Tarts with a warm cup of tea or coffee!