Preheat oven to 450 °F. Halve potatoes and add to boiling water that has been salted. Boil potatoes for 15 minutes or until fork tender. Drain potatoes and let dry briefly.
In a small saucepan, combine butter, olive oil, rosemary, salt and pepper and stir until melted and combined. Don't overcook. Remove from heat once combined.
Add rosemary butter to a large mixing bowl and add in boiled potatoes. Toss to combine. Remove potatoes and place on a baking sheet with parchment paper, cut side up. Bake at 450F for 10 minutes.
Remove potatoes and toss back in rosemary butter. Toss well and return to baking sheet, this time cut-side down. Roast for another 10-15 minutes.
Remove potatoes and serve with other dishes of your choice.