1.For mandarin jam, stir mandarin, sugar, lemon juice and vanilla bean and seeds in a large saucepan over medium-high heat until sugar dissolves. Bring to the boil and cook, stirring occasionally, until jammy (25-30 minutes). Transfer to a container and cool to room temperature. Store jam in an airtight container at room temperature for up to 3 days.

2.Mix flour, yeast and 100gm sugar in an electric mixer fitted with a dough hook to combine. Add milk, eggs, yolk, mandarin rind and juice and a pinch of salt, and knead until a dough forms. With the motor running, add 150gm butter, a little at a time, kneading until incorporated between additions. Continue to knead until a smooth dough forms. Transfer to a lightly buttered bowl, cover with plastic wrap and stand at room temperature until doubled in size (1-1½ hours).

3.Meanwhile, preheat oven to 180C. Butter a 15cm x 30cm rectangular cake tin and line with baking paper. Beat remaining butter and sugar in a bowl until pale (1-2 minutes), then beat in almond meal and cinnamon until smooth.

4.Turn dough out onto a lightly floured surface, knock back, then roll out to a 40cm square. Spread almond mixture over dough, leaving a 4cm-border, then spread half the mandarin jam on top. Roll dough away from you to form a cylinder, then halve the cylinder lengthways and turn each piece cut-side up. Twist pieces over one another and place in prepared tin. Cover loosely with plastic wrap or a clean tea towel and stand in a warm place until risen by half (35-40 minutes).

5.Brush babka with eggwash and bake until golden brown and a skewer inserted withdraws clean (40-45 minutes; cover with foil if browning too quickly). Cool in tin for 10 minutes, then turn out onto a wire rack. Dust with icing sugar and serve warm or toasted with mandarin jam. Babka will keep in an airtight container for 3 days.