Melt butter, sugar and rum in a small saucepan. Grill pineapple on both sides until golden brown, about 2 to 3 minutes per side. Spoon rum glaze over grilled pineapple.
Whisk together mascarpone and vanilla seeds. Top each slice of pineapple with a dollop of vanilla mascarpone. Garnish with a few fresh blueberries.
Recipe courtesy of Bobby Flay