Preheat oven to 190C and line two baking trays with baking paper. Sieve flour, 1 tbsp caster sugar and a pinch of salt into a bowl. Whisk yolks and eggs and remaining caster sugar in an electric mixer on medium-high speed until tripled in volume (7-8 minutes), then whisk in vanilla and transfer to a large bowl. Add flour mixture in three batches, gently folding between additions, then fold in butter. Spoon into a piping bag fitted with a 12mm plain nozzle and pipe 8cm-long fingers onto prepared trays. Dust heavily with icing sugar, then bake until light golden, and firm yet spongy when lightly pressed (7-8 minutes). Transfer to a wire rack to cool. These will keep in an airtight container for up to a week. Makes about 36 individual sponge fingers.
For vanilla buttercream, beat ingredients in an electric mixer until light and fluffy (10 minutes).
Line a tray with baking paper. Melt chocolate in a bowl over a saucepan of simmering water, add vegetable oil and stir until smooth. Spread buttercream over bases of half the sponge fingers and jam over the other half. Sandwich together one of each, then half-dip in chocolate and place on prepared tray to set (2 hours). Store in an airtight container for up to 3 days.