614]. Then we construct a (2 + 1)-dimensional double central extension of the cotangent universal hierarchy and show that this extension is bi-Hamiltonian. This yields, as a byproduct, the central extension of the original universal hierarchy.The so-called universal hierarchy [1][2][3][4][5] is now a subject of intense research. This hierarchy is, in its original form, an infinite hierarchy of coupled dispersionless (1 + 1)-dimensional integrable systems. The universal hierarchy can be thought of as a model equation of soliton theory from which one can obtain many well-known and new soliton equations upon imposing suitable differential constraints. It is natural to ask whether we can construct a (2 + 1)-dimensional extension of this hierarchy which could yield, upon imposing suitable constraints, (2 + 1)-dimensional integrable systems.However, there appears to be no straightforward way to include the universal hierarchy into the standard Lie-algebraic R-matrix scheme in spirit of [6][7][8][9][10][11] which would enable one to construct the (2 + 1)-dimensional extension of the hierarchy in question and prove integrability thereof using the central extension approach.To circumvent this difficulty, we first lift, in spirit of [12][13][14], the universal hierarchy (3) to the cotangent universal hierarchy, see Eq. (14) below. This cotangent universal hierarchy is already amenable to the approach of [7-9], and integrability of central extensions of (14) follows from the general theory presented there.In fact, we go even further than that. Motivated by [13,14], we perform the double central extension of (14) using two cocycles rather than one. Commutativity of so constructed flows and integrability of the resulting hierarchy (27) still follows from the general results of the R-matrix scheme of [7][8][9]. The first of the cocycles in question, (16), introduces the second space variable y, thus yielding a (2 + 1)-dimensional rather than (1 + 1)-dimensional hierarchy, while the second cocycle, (17), introduces dispersion. A quite different way of introducing dispersion that leads to infinite-order differential equations can be found in [15].What is more, (27) contains a subhierarchy (28) which is precisely the central extension of the original universal hierarchy (3), and integrability of (28) follows from that of (27).The hierarchy (27) is bi-Hamiltonian, and, quite interestingly, the Poisson brackets (22) of (27) are not of the operand type, see e.g. [10, [16][17][18] and references therein for the latter, but rather of the same type that occurs in (1 + 1) dimensions and also for (2 + 1)-dimensional hydrodynamic-type systems [11], as is explicitly revealed by Examples 1 and 2. The corresponding recursion operators, being the ratios of