Place chopped orange slice candy in a heatproof bowl, pour boiling water over it, stir, and let sit about 10 minutes until softened and syrupy
Preheat oven to 325°F (165°C) and grease a bundt pan or 9×13-inch pan
Cream butter and granulated sugar together until light and fluffy
Add eggs one at a time, mixing well after each
Whisk flour, baking soda, and salt together in a separate bowl
Add dry ingredients to the butter mixture alternately with buttermilk, mixing just until combined
Stir in softened orange slice candy with its syrup and add vanilla extract
Pour batter into prepared pan and smooth the top
Bake 60–70 minutes for a bundt pan or 45–55 minutes for a 9×13-inch pan, until a toothpick comes out clean
Cool in pan for 15 minutes, then turn out and cool completely
Whisk powdered sugar, orange juice, and vanilla to make glaze
Drizzle glaze over cooled cake