Grease and line a 33 x 23cm Swiss roll tin and lightly grease the lining paper.
Sieve the flour with the cocoa and baking powder.
Place the eggs with the sugar in a roomy, heat-resistant bowl and place over a pan of gently steaming water, making sure the bottom of the bowl is not touching the water. Whisk until the mixture thickens and the mixture leaves a trail when lifted out.
Remove from the hot water and continue whisking until the mixture has cooled. Lightly fold in the sieved flour mixture and place in the prepared tin, smoothing the mixture well into the corners.
Bake for about 10 minutes or until firm to the touch and beginning to shrink from the sides of the tin.
Turn out quickly onto a sheet of lightly sugared greaseproof paper, remove the lining paper, trim off the outside edges of the sponge, then immediately replace the tin on top.
Cover with a damp cloth and leave to become quite cold.
For the filling, cream the butter until light and fluffy, then add the sieved icing sugar a little at a time together with the ground almonds and vanilla flavouring. Beat well then spread the filling evenly over the cold Swiss roll to within 5mm of the edge. Roll up firmly from the short edge and remove from the greaseproof paper.