Preheat oven to 180°C (gas mark 4)
Line a 20cm springform tin with parchment and butter the sides
Place 1 tbsp caster sugar in the base of the tin and shake to spread
Trim the rhubarb into 2.5cm sticks and arrange in a single layer
Sprinkle the remaining 1 tbsp caster sugar over rhubarb and set aside
In a mixing bowl, cream 175g butter and 175g caster sugar for 3-4 minutes until pale and fluffy
Add eggs one at a time, beating well after each addition
Sift in flour, custard powder, and sea salt; gently fold in
Stir in chilled soured cream
Scrape the batter into the prepared tin and spread it to cover the rhubarb
Bake for 45-55 minutes until the top springs back when lightly touched
Check with a skewer – it should come out mostly clean with just a few crumbs
Cool completely in the tin (at least 45 minutes) before inverting the cake onto a serving plate
To make the glaze, simmer milk, tea bags, and vanilla extract until steaming hot
Remove tea bags and whisk 35g custard powder with 1 tbsp chilled milk
Add the custard powder mixture to the hot milk and stir until smooth and shiny
Allow glaze to cool slightly before drizzling over the top of the cooled cake
Serve slightly warm or at room temperature