In bowl, whisk together flour, baking powder and soda, and salt. Grease two 9-inch/23 cm round cake pans. Line bottoms with parchment paper; flour sides.

In large bowl, beat butter until light and creamy. Beat in sugar, 1/3 cup (75 mL) at a time, on high speed until light and fluffy, about 3 minutes. Beat in eggs, 1 at a time; beat in rum and vanilla (mixture may look curdled but will come together). Gradually mix in flour mixture and buttermilk, making three additions of flour mixture and two of buttermilk.

Divide between cake pans, smoothing tops. Bake in centre of 350F/180C oven until tester inserted in centre comes out clean, 35 to 40 minutes.

Let cool on rack for 10 minutes. Slide knife around edges and remove from pans; peel off paper. Let cool on rack.

Molasses Buttercream: In heatproof bowl, whisk egg whites with sugar until frothy. Place over saucepan of boiling water; cook, whisking, until thermometer registers 110F/43C or finger can remain in mixture for no longer than 10 seconds, 1 to 2 minutes.

Beat with heavy-duty or stand mixer until cool, 7 to 10 minutes. Beat in butter, 1/4 cup (60 mL) at a time, until silky; beat in molasses and vanilla. Trim top of cake layers until level; spread with buttercream.